diff --git a/lib/6to5/transformation/modules/common.js b/lib/6to5/transformation/modules/common.js
index fa2c7263fb..341846f4a0 100644
--- a/lib/6to5/transformation/modules/common.js
+++ b/lib/6to5/transformation/modules/common.js
@@ -9,8 +9,10 @@ function CommonJSFormatter(file) {
   DefaultFormatter.apply(this, arguments);
 
   var hasNonDefaultExports = false;
-  traverse(file.ast, function (node) {
-    if (t.isExportDeclaration(node) && !node.default) hasNonDefaultExports = true;
+  traverse(file.ast, {
+    enter: function (node) {
+      if (t.isExportDeclaration(node) && !node.default) hasNonDefaultExports = true;
+    }
   });
   this.hasNonDefaultExports = hasNonDefaultExports;
 }
@@ -31,9 +33,6 @@ CommonJSFormatter.prototype.importSpecifier = function (specifier, node, nodes)
     ]));
   } else {
     // import foo from "foo";
-    if (specifier.default) {
-      specifier.id = t.identifier("default");
-    }
 
     var templateName = "require-assign";
 
diff --git a/lib/6to5/transformation/modules/system.js b/lib/6to5/transformation/modules/system.js
index 73ddc6f107..d2dc31dc3b 100644
--- a/lib/6to5/transformation/modules/system.js
+++ b/lib/6to5/transformation/modules/system.js
@@ -5,7 +5,6 @@ var t    = require("../../types");
 var _    = require("lodash");
 
 var SETTER_MODULE_NAMESPACE = t.identifier("m");
-var DEFAULT_IDENTIFIER      = t.identifier("default");
 var NULL_SETTER             = t.literal(null);
 
 function SystemFormatter(file) {
@@ -117,9 +116,6 @@ SystemFormatter.prototype.importSpecifier = function (specifier, node) {
   var variableName = t.getSpecifierName(specifier);
 
   // import foo from "foo";
-  if (specifier.default) {
-    specifier.id = DEFAULT_IDENTIFIER;
-  }
 
   var MODULE_NAME = node.source.value;
 
diff --git a/lib/6to5/transformation/transformers/_alias-functions.js b/lib/6to5/transformation/transformers/_alias-functions.js
index 3ea43351a4..4d4b78ea67 100644
--- a/lib/6to5/transformation/transformers/_alias-functions.js
+++ b/lib/6to5/transformation/transformers/_alias-functions.js
@@ -15,38 +15,42 @@ var go = function (getBody, node, file, scope) {
 
   // traverse the function and find all alias functions so we can alias
   // `arguments` and `this` if necessary
-  traverse(node, function (node) {
-    if (!node._aliasFunction) {
-      if (t.isFunction(node)) {
-        // stop traversal of this node as it'll be hit again by this transformer
-        return false;
-      } else {
-        return;
+  traverse(node, {
+    enter: function (node) {
+      if (!node._aliasFunction) {
+        if (t.isFunction(node)) {
+          // stop traversal of this node as it'll be hit again by this transformer
+          return false;
+        } else {
+          return;
+        }
       }
+
+      // traverse all child nodes of this function and find `arguments` and `this`
+      traverse(node, {
+        enter: function (node, parent) {
+          if (t.isFunction(node) && !node._aliasFunction) {
+            return false;
+          }
+
+          if (node._ignoreAliasFunctions) return false;
+
+          var getId;
+
+          if (t.isIdentifier(node) && node.name === "arguments") {
+            getId = getArgumentsId;
+          } else if (t.isThisExpression(node)) {
+            getId = getThisId;
+          } else {
+            return;
+          }
+
+          if (t.isReferenced(node, parent)) return getId();
+        }
+      });
+
+      return false;
     }
-
-    // traverse all child nodes of this function and find `arguments` and `this`
-    traverse(node, function (node, parent) {
-      if (t.isFunction(node) && !node._aliasFunction) {
-        return false;
-      }
-
-      if (node._ignoreAliasFunctions) return false;
-
-      var getId;
-
-      if (t.isIdentifier(node) && node.name === "arguments") {
-        getId = getArgumentsId;
-      } else if (t.isThisExpression(node)) {
-        getId = getThisId;
-      } else {
-        return;
-      }
-
-      if (t.isReferenced(node, parent)) return getId();
-    });
-
-    return false;
   });
 
   var body;
diff --git a/lib/6to5/transformation/transformers/es6-classes.js b/lib/6to5/transformation/transformers/es6-classes.js
index 71e3e2680a..383a477d3d 100644
--- a/lib/6to5/transformation/transformers/es6-classes.js
+++ b/lib/6to5/transformation/transformers/es6-classes.js
@@ -247,17 +247,19 @@ Class.prototype.replaceInstanceSuperReferences = function (methodNode) {
   var method = methodNode.value;
   var self = this;
 
-  traverse(method, function (node, parent) {
-    if (t.isIdentifier(node, { name: "super" })) {
-      return self.superIdentifier(methodNode, node, parent);
-    } else if (t.isCallExpression(node)) {
-      var callee = node.callee;
-      if (!t.isMemberExpression(callee)) return;
-      if (callee.object.name !== "super") return;
+  traverse(method, {
+    enter: function (node, parent) {
+      if (t.isIdentifier(node, { name: "super" })) {
+        return self.superIdentifier(methodNode, node, parent);
+      } else if (t.isCallExpression(node)) {
+        var callee = node.callee;
+        if (!t.isMemberExpression(callee)) return;
+        if (callee.object.name !== "super") return;
 
-      // super.test(); -> ClassName.prototype.MethodName.call(this);
-      callee.property = t.memberExpression(callee.property, t.identifier("call"));
-      node.arguments.unshift(t.thisExpression());
+        // super.test(); -> ClassName.prototype.MethodName.call(this);
+        callee.property = t.memberExpression(callee.property, t.identifier("call"));
+        node.arguments.unshift(t.thisExpression());
+      }
     }
   });
 };
diff --git a/lib/6to5/transformation/transformers/es6-constants.js b/lib/6to5/transformation/transformers/es6-constants.js
index 4266ceca44..a3021188fb 100644
--- a/lib/6to5/transformation/transformers/es6-constants.js
+++ b/lib/6to5/transformation/transformers/es6-constants.js
@@ -69,12 +69,14 @@ exports.ForStatement = function (node, parent, file) {
 
   if (!hasConstants) return;
 
-  traverse(node, function (child, parent, scope) {
-    if (child._ignoreConstant) return;
-    if (t.isVariableDeclaration(child)) return;
+  traverse(node, {
+    enter: function (child, parent, scope) {
+      if (child._ignoreConstant) return;
+      if (t.isVariableDeclaration(child)) return;
 
-    if (t.isVariableDeclarator(child) || t.isDeclaration(child) || t.isAssignmentExpression(child)) {
-      check(parent, getIds(child), scope);
+      if (t.isVariableDeclarator(child) || t.isDeclaration(child) || t.isAssignmentExpression(child)) {
+        check(parent, getIds(child), scope);
+      }
     }
   });
 };
diff --git a/lib/6to5/transformation/transformers/es6-default-parameters.js b/lib/6to5/transformation/transformers/es6-default-parameters.js
index 93b0ef284c..9715c2bbdc 100644
--- a/lib/6to5/transformation/transformers/es6-default-parameters.js
+++ b/lib/6to5/transformation/transformers/es6-default-parameters.js
@@ -37,7 +37,7 @@ exports.Function = function (node, parent, file, scope) {
       };
 
       check(def, node);
-      traverse(def, check);
+      traverse(def, { enter: check });
     });
 
     // we're accessing a variable that's already defined within this function
diff --git a/lib/6to5/transformation/transformers/es6-let-scoping.js b/lib/6to5/transformation/transformers/es6-let-scoping.js
index 625159fce6..7d6fa09b76 100644
--- a/lib/6to5/transformation/transformers/es6-let-scoping.js
+++ b/lib/6to5/transformation/transformers/es6-let-scoping.js
@@ -176,7 +176,7 @@ LetScoping.prototype.remap = function () {
 
   var traverseReplace = function (node, parent) {
     replace(node, parent);
-    traverse(node, replace);
+    traverse(node, { enter: replace });
   };
 
   var loopParent = this.loopParent;
@@ -186,7 +186,7 @@ LetScoping.prototype.remap = function () {
     traverseReplace(loopParent.update, loopParent);
   }
 
-  traverse(block, replace);
+  traverse(block, { enter: replace });
 };
 
 /**
@@ -275,33 +275,35 @@ LetScoping.prototype.checkLoop = function () {
     hasBreak:    false,
   };
 
-  traverse(this.block, function (node, parent) {
-    var replace;
+  traverse(this.block, {
+    enter: function (node, parent) {
+      var replace;
 
-    if (t.isFunction(node) || t.isLoop(node)) {
-      return false;
-    }
-
-    if (node && !node.label) {
-      if (t.isBreakStatement(node)) {
-        if (t.isSwitchCase(parent)) return;
-
-        has.hasBreak = true;
-        replace = t.returnStatement(t.literal("break"));
-      } else if (t.isContinueStatement(node)) {
-        has.hasContinue = true;
-        replace = t.returnStatement(t.literal("continue"));
+      if (t.isFunction(node) || t.isLoop(node)) {
+        return false;
       }
-    }
 
-    if (t.isReturnStatement(node)) {
-      has.hasReturn = true;
-      replace = t.returnStatement(t.objectExpression([
-        t.property("init", t.identifier("v"), node.argument || t.identifier("undefined"))
-      ]));
-    }
+      if (node && !node.label) {
+        if (t.isBreakStatement(node)) {
+          if (t.isSwitchCase(parent)) return;
 
-    if (replace) return t.inherits(replace, node);
+          has.hasBreak = true;
+          replace = t.returnStatement(t.literal("break"));
+        } else if (t.isContinueStatement(node)) {
+          has.hasContinue = true;
+          replace = t.returnStatement(t.literal("continue"));
+        }
+      }
+
+      if (t.isReturnStatement(node)) {
+        has.hasReturn = true;
+        replace = t.returnStatement(t.objectExpression([
+          t.property("init", t.identifier("v"), node.argument || t.identifier("undefined"))
+        ]));
+      }
+
+      if (replace) return t.inherits(replace, node);
+    }
   });
 
   return has;
@@ -314,19 +316,21 @@ LetScoping.prototype.checkLoop = function () {
 
 LetScoping.prototype.hoistVarDeclarations = function () {
   var self = this;
-  traverse(this.block, function (node, parent) {
-    if (t.isForStatement(node)) {
-      if (isVar(node.init, node)) {
-        node.init = t.sequenceExpression(self.pushDeclar(node.init));
+  traverse(this.block, {
+    enter: function (node, parent) {
+      if (t.isForStatement(node)) {
+        if (isVar(node.init, node)) {
+          node.init = t.sequenceExpression(self.pushDeclar(node.init));
+        }
+      } else if (t.isFor(node)) {
+        if (isVar(node.left, node)) {
+          node.left = node.left.declarations[0].id;
+        }
+      } else if (isVar(node, parent)) {
+        return self.pushDeclar(node).map(t.expressionStatement);
+      } else if (t.isFunction(node)) {
+        return false;
       }
-    } else if (t.isFor(node)) {
-      if (isVar(node.left, node)) {
-        node.left = node.left.declarations[0].id;
-      }
-    } else if (isVar(node, parent)) {
-      return self.pushDeclar(node).map(t.expressionStatement);
-    } else if (t.isFunction(node)) {
-      return false;
     }
   });
 };
@@ -359,31 +363,35 @@ LetScoping.prototype.getLetReferences = function () {
 
   // traverse through this block, stopping on functions and checking if they
   // contain any outside let references
-  traverse(this.block, function (node, parent, scope) {
-    if (t.isFunction(node)) {
-      traverse(node, function (node, parent) {
-        // not an identifier so we have no use
-        if (!t.isIdentifier(node)) return;
+  traverse(this.block, {
+    enter: function (node, parent, scope) {
+      if (t.isFunction(node)) {
+        traverse(node, {
+          enter: function (node, parent) {
+            // not an identifier so we have no use
+            if (!t.isIdentifier(node)) return;
 
-        // not a direct reference
-        if (!t.isReferenced(node, parent)) return;
+            // not a direct reference
+            if (!t.isReferenced(node, parent)) return;
 
-        // this scope has a variable with the same name so it couldn't belong
-        // to our let scope
-        if (scope.hasOwn(node.name)) return;
+            // this scope has a variable with the same name so it couldn't belong
+            // to our let scope
+            if (scope.hasOwn(node.name)) return;
 
-        closurify = true;
+            closurify = true;
 
-        // this key doesn't appear just outside our scope
-        if (!_.contains(self.info.outsideKeys, node.name)) return;
+            // this key doesn't appear just outside our scope
+            if (!_.contains(self.info.outsideKeys, node.name)) return;
 
-        // push this badboy
-        self.letReferences[node.name] = node;
-      });
+            // push this badboy
+            self.letReferences[node.name] = node;
+          }
+        });
 
-      return false;
-    } else if (t.isLoop(node)) {
-      return false;
+        return false;
+      } else if (t.isLoop(node)) {
+        return false;
+      }
     }
   });
 
diff --git a/lib/6to5/transformation/transformers/playground-object-getter-memoization.js b/lib/6to5/transformation/transformers/playground-object-getter-memoization.js
index 8c3c835023..560f2967c3 100644
--- a/lib/6to5/transformation/transformers/playground-object-getter-memoization.js
+++ b/lib/6to5/transformation/transformers/playground-object-getter-memoization.js
@@ -21,11 +21,13 @@ exports.MethodDefinition = function (node, parent, file, scope) {
   var memoId = t.memberExpression(t.thisExpression(), t.identifier(key));
   var doneId = t.memberExpression(t.thisExpression(), t.identifier(key + "Done"));
 
-  traverse(value, function (node) {
-    if (t.isFunction(node)) return;
+  traverse(value, {
+    enter: function (node) {
+      if (t.isFunction(node)) return;
 
-    if (t.isReturnStatement(node) && node.argument) {
-      node.argument = t.assignmentExpression("=", memoId, node.argument);
+      if (t.isReturnStatement(node) && node.argument) {
+        node.argument = t.assignmentExpression("=", memoId, node.argument);
+      }
     }
   });
 
diff --git a/lib/6to5/traverse/index.js b/lib/6to5/traverse/index.js
index aea63764e0..fcbdf1f587 100644
--- a/lib/6to5/traverse/index.js
+++ b/lib/6to5/traverse/index.js
@@ -4,14 +4,14 @@ var Scope = require("./scope");
 var t     = require("../types");
 var _     = require("lodash");
 
-function traverse(parent, callbacks, opts) {
+function traverse(parent, opts, scope) {
   // falsy node
   if (!parent) return;
 
   // array of nodes
   if (_.isArray(parent)) {
     _.each(parent, function (node) {
-      traverse(node, callbacks, opts);
+      traverse(node, opts, scope);
     });
     return;
   }
@@ -21,13 +21,6 @@ function traverse(parent, callbacks, opts) {
   if (!keys) return;
 
   opts = opts || {};
-  if (_.isArray(opts)) opts = { blacklist: opts };
-
-  // blacklist these node types from being traversed
-  var blacklistTypes = opts.blacklist || [];
-
-  // normalise callbacks
-  if (_.isFunction(callbacks)) callbacks = { enter: callbacks };
 
   for (var i in keys) {
     var key = keys[i];
@@ -41,7 +34,7 @@ function traverse(parent, callbacks, opts) {
       if (!node) return;
 
       // type is blacklisted
-      if (blacklistTypes.indexOf(node.type) > -1) return;
+      if (opts.blacklist && opts.blacklist.indexOf(node.type) > -1) return;
 
       // replace node
       var maybeReplace = function (result) {
@@ -58,12 +51,12 @@ function traverse(parent, callbacks, opts) {
       };
 
       //
-      var opts2 = { scope: opts.scope, blacklist: opts.blacklist };
-      if (t.isScope(node)) opts2.scope = new Scope(node, opts.scope);
+      var ourScope = scope;
+      if (t.isScope(node)) ourScope = new Scope(node, scope);
 
       // enter
-      if (callbacks.enter) {
-        var result = callbacks.enter(node, parent, opts2.scope);
+      if (opts.enter) {
+        var result = opts.enter(node, parent, ourScope);
         maybeReplace(result);
 
         // stop iteration
@@ -71,11 +64,11 @@ function traverse(parent, callbacks, opts) {
       }
 
       // traverse node
-      traverse(node, callbacks, opts2);
+      traverse(node, opts, ourScope);
 
       // exit
-      if (callbacks.exit) {
-        maybeReplace(callbacks.exit(node, parent, opts2.scope));
+      if (opts.exit) {
+        maybeReplace(opts.exit(node, parent, ourScope));
       }
     };
 
@@ -113,7 +106,7 @@ traverse.removeProperties = function (tree) {
   };
 
   clear(tree);
-  traverse(tree, clear);
+  traverse(tree, { enter: clear });
 
   return tree;
 };
@@ -129,12 +122,15 @@ traverse.hasType = function (tree, type, blacklistTypes) {
   // the type we're looking for is the same as the passed node
   if (tree.type === type) return true;
 
-  traverse(tree, function (node) {
-    if (node.type === type) {
-      has = true;
-      return false;
+  traverse(tree, {
+    blacklist: blacklistTypes,
+    enter: function (node) {
+      if (node.type === type) {
+        has = true;
+        return false;
+      }
     }
-  }, { blacklist: blacklistTypes });
+  });
 
   return has;
 };
diff --git a/lib/6to5/traverse/scope.js b/lib/6to5/traverse/scope.js
index 603a33a823..e5acde3244 100644
--- a/lib/6to5/traverse/scope.js
+++ b/lib/6to5/traverse/scope.js
@@ -82,31 +82,33 @@ Scope.prototype.getInfo = function () {
   // Program, Function - var variables
 
   if (t.isProgram(block) || t.isFunction(block)) {
-    traverse(block, function (node, parent, scope) {
-      if (t.isFor(node)) {
-        _.each(FOR_KEYS, function (key) {
-          var declar = node[key];
-          if (t.isVar(declar)) add(declar);
-        });
+    traverse(block, {
+      enter: function (node, parent, scope) {
+        if (t.isFor(node)) {
+          _.each(FOR_KEYS, function (key) {
+            var declar = node[key];
+            if (t.isVar(declar)) add(declar);
+          });
+        }
+
+        // this block is a function so we'll stop since none of the variables
+        // declared within are accessible
+        if (t.isFunction(node)) return false;
+
+        // function identifier doesn't belong to this scope
+        if (block.id && node === block.id) return;
+
+        if (t.isIdentifier(node) && t.isReferenced(node, parent) && !scope.has(node.name)) {
+          add(node, true);
+        }
+
+        // we've ran into a declaration!
+        // we'll let the BlockStatement scope deal with `let` declarations unless
+        if (t.isDeclaration(node) && !t.isLet(node)) {
+          add(node);
+        }
       }
-
-      // this block is a function so we'll stop since none of the variables
-      // declared within are accessible
-      if (t.isFunction(node)) return false;
-
-      // function identifier doesn't belong to this scope
-      if (block.id && node === block.id) return;
-
-      if (t.isIdentifier(node) && t.isReferenced(node, parent) && !scope.has(node.name)) {
-        add(node, true);
-      }
-
-      // we've ran into a declaration!
-      // we'll let the BlockStatement scope deal with `let` declarations unless
-      if (t.isDeclaration(node) && !t.isLet(node)) {
-        add(node);
-      }
-    }, { scope: this });
+    }, this);
   }
 
   // Function - params, rest
diff --git a/lib/6to5/util.js b/lib/6to5/util.js
index 1478340ae8..6afcdef2c7 100644
--- a/lib/6to5/util.js
+++ b/lib/6to5/util.js
@@ -150,9 +150,11 @@ exports.template = function (name, nodes, keepExpression) {
   template = _.cloneDeep(template);
 
   if (!_.isEmpty(nodes)) {
-    traverse(template, function (node) {
-      if (t.isIdentifier(node) && _.has(nodes, node.name)) {
-        return nodes[node.name];
+    traverse(template, {
+      enter: function (node) {
+        if (t.isIdentifier(node) && _.has(nodes, node.name)) {
+          return nodes[node.name];
+        }
       }
     });
   }