There is always talk about a points system for the forum, and countless extensions built and abandoned.
A points system like "1 point per post" or "reputation" would be a welcome feature in phpBB that I feel a lot of other forums have already.
Generally, points systems are quite big and complex, and if you use an extension, it usually is broken by the next big phpBB update.
It's the main thing that it's missing imo!
I love teh phpBB