Something sort of random I just noticed: you can wrap stuff in {} to make blocks without an actual block statement...just a random block of code. That may be pretty useful once the let keyword becomes widely available, and could be used now to maybe organize lots of code?
"let" has worked in Firefox for years; support for it first shipped in Firefox 2 back in 2006. You do have to opt in to it by putting type="application/javascript;version=1.7" on your script tag, since the feature is not backwards-compatible and hence couldn't be added to existing scripts without possibly breaking them.
JS 1.7 in general added a bunch of neat features (array comprehensions, destructuring assignment, etc) that are only now slowly making their way to specifications after some foot-dragging by certain parties. See https://developer.mozilla.org/en/JavaScript/New_in_JavaScrip...