My faves are:
- Having a native module system with import/export capabilities (no more IIFEs or UMD ceremony). Unfortunately Node JS still hasn’t caught up with this change properly (but it’s coming). I use Babel when working with Node.
- The introduction of `const` and `let` and their ability to preserve scope and in the case of `const` prevent re-assignment.
- The introduction of arrow functions (I loved them from my C# days)
- Native support for Promises