This one is super-useful. I'd rather have it standardized, in a slightly improved form that allows custom classes to easily be iterated.

A slightly different form, |for (val of obj)|, got standardized. To use it you define obj[@@iterator] as a function that returns an iterator object yielding values that are bound in sequence to val. @@iterator is -- very shortly -- going to be Symbol.iterator. Right now it's just "@@iterator", I think, as a temporary hack. So enabling symbol support in releases, and making Symbol.iterator work, are prerequisites to using for-of on objects other than those that we've already made iterable. Arrays are the most common already-iterable object right now.