Internally we use jstransform (github.com/facebook/jstransform) because it allows us to plug in individual transforms more easily. It's also what React uses for

Internally we use jstransform (github.com/facebook/jstransform) because it allows us to plug in individual transforms more easily. It's also what React uses for it's jsx transformations. jstransform also ships with a few es6 features out of the box (classes, arrow functions, etc -- and the number is growing!), but Traceur definitely has more.