[is:unanswered] [python] - find unanswered Python questions, this is somewhat possible by clicking "Unanswered" and appending /tagged/python but you cannot search the question body/title, only filter by tags
[subjective] -[is:communitywiki] - find subjective questions not community-wiki'd (For the "Community Wiki Police"!..)
[.net] [is:answer] [votes:>100] - find popular answers to .net tagged questions
[is:flagged] [votes:>10] - find popular questions that have been flagged
[not-programming-related] [votes:<0] - find non programming related questions that have been downvoted
[vs*] - wildcard tag search. Searches for all tags starting with vs - to find questions tagged with vs2008, vs2010 etc (idea from here)
I like the idea of user filters. Compose your own rules and have the ability of 'saved' searches. (I always wanted a search option for truly unanswered questions). (Here are one and two screenshots how we did it once.)
You know what would really rock? Taking Google's search operators and formalizing them into an international standard that anyone who wanted to do "search" could then implement (poorly). And from then taking that standard and formalizing how to extend it. Then I could tag Google's superior engine and use it to search StackOverflow within a specific tag like I can with the lackluster index on the site itself.
I know this is going to sound very insecure (SQL Injection) but why not allow users of SO to search using a SQL like syntax. Most of us are programmers anyway, which also makes this a good learning tool.