I'd love if this was implemented so that if I type "[" in the search box, the same autocomplete stuff that fills in the tags for you when you ask a question, fills them in in the search box (preserving the [, of course, and possibly adding a ] at the end of each tag).
Since the autocompletion for the tag field in the ask question form is already implemented, this shouldn't be big at all. But it would make it a LOT easier to search (successfully) by tags. Only today I found a total of three questions tagged [integration-tests] and over 500 tagged [integration-testing] - with autocomplete I would've known how many hits each version would have even before i searched.