i think questions which don't yet have a selected answer would be best.
and have you thought of a search so that people don't questions repeatedly? (ability to completley delete answers and questions would be nice to but only admins or the poster of the question/answer should be allowed to take this action) i would love to see those things implemented! anyways, love to see a php Question and Answer that actually works well! good job!
Edit: oh yeah and ability to report users would be great, i currently use the feedback form for it but it doesn't seem to work as well, so a report center in the acp to review all reported posts (available to editors to so more like a editor/moderator control panel) but not an email form, and possibly notification of new reports when you log in. a private messaging system would also be nice but i don't know if there is sufficient demand for that, plus it may also be to much like a forum.
anyway thanks for the help and good-luck! :)