Suffice it to say, the result is a language that is almost (but not quite) completely unlike Javascript. It's a superset, and broadly back-compatible, but really startlingly different in what a normal program looks like. In many ways it's a much more conventional and ordinary object-oriented programming language, but with occasional surprising twists and turns. (For instance, look up the "like" and "wrap" operators.)
ActionScript programmers won't find as much surprising here, since AS3 spun off from the discussion of ES4. But even there, you'll find a lot that's new: there's almost as much enhancement from AS3 to ES4 as there was from old-style Javascript to AS3...
