I think it would be nice to be able to do an advanced search, such as searching for posts with fewer than/more than X number of comments, Karma points, etc., or excluding new users, including an arbitrary list of users, etc. in addition to just keyword searches.
I'm not sure how many of those advanced features are worth the effort of developing -
fewer than/more than: I don't see the benefit of this except for threads that are really active, which easily could include slight flame wars and off topic discussions
karma points: perhaps useful
excluding new users: don't see the point - throw away accounts are popular here and tend to come up frequently - I can't think of a single scenario where I would ever want to filter threads like this
including list of users: somewhat useful, if going to a users profile and filtering by a single user wouldn't be good enough
the only thing I could see as a useful advanced search option would be url filter to find posts about certain domains.
Granted these are just opinions on how I would use a HN advanced search feature; so my opinion could be part of the minority.
site:news.ycombinator.com <whatever-your-query-is>