I'd like to propose following enhancements to forum search:
1. Look only for questions that where answered or has a (very) helpful answers (btw, this could be a result sorting option as well) -- I guess reasons are obvious.
2. Search only in original questions (no replies) -- sometimes replies produces more noise then help
3. Search by conversation -- display only topics where me and given user participates / I replied to question of given user / User replied to my questions.