I'd like to see the ability to rank messages (e.g. +1/-1; thumbs up/thumbs down).
Personally, while I like the ability to rate a "Post Topic" on a 1 to 5 "star" scale, I'd find it helpful to be able to find and/or sort responses to the initial post based upon the "helpfulness" of the replies. It would certainly be helpful with topics that have many pages of replies, and the discussion has gone off from the original topic.
Apologies if this has been suggested before, I simply haven't seen it.