This blog post originally appeared at Skyline Technologies (my employer) http://skylinetechnologies.com/Blog/Article/2388/ECMAScript-6.aspx
Argue all you want about which web application framework (ASP.NET, Ruby on Rails, PHP, etc.) is better, but at the end of the day, you will be forced to agree on one thing:
Let's take a peek.
default function parameters: Function parameters receive a default value if no parameter is passed.
let: Block scoped variables that are not subject to variable hoisting.
const: Read-only variables.
for...of: Convenient way of doing iteration over enumerables.
maps and sets: Maps are objects containing key/value pairs and Sets are objects that hold unique values.
Finally, if you're looking to play around with these right this minute, Firefox tries to stay ahead of the curve. Be forewarned, however, as these are implementations of features for a specification that has not be finalized.