The new syntax makes ECMAScript more like traditional programming languages, but the language also aims to stay backwards compatible with the current version. In other words, you can build classes using the new
class syntax or you may use prototype like JS developers do today (sort of like how AS1 and AS2 gave us these same options in Flash for a long time). It raises a question in many minds, “Why?”.
Microsoft JScript blog: ECMAScript 3 and beyond
Microsoft’s Chris Wilson: What I think about ES4.
Brendan Eich: Open Letter to Chris Wilson
Update: New information about this topic is available in my article How will ECMAScript “Harmony” affect ActionScript 3.