I think the social factor is the key of maintaining a lively community.
So I compared q2a quickly to social networks, and found out that q2a is missing one important feature: Users cannot post on user pages (user profile pages), to say thank you, to leave a request on xyz, to kind of chat, to leave a status ...
So if anyone is advanced enough, this could be your next plugin, among others I think of NoahY and Scott :)
Have a good week-end!
extra tags: guestbook, user status