This is a take off Facebook but I think a useful thing would be the ability to tag a username to a thread so that it pops up (as far as the app goes) in there current/participated section to being there attention to a particular subject matter

Example: someone asks a question pertaining to a particular subject matter that you don't know much on but happen to know a username of someone very knowledgable in the matter, you tag them; they come in and give there $0.02 saving the answer from going to PM, and the same question being asked later.