1. Functionality to lock question after certain no. of days (It depends of the admin what no. of days he wants after which user can not answer the questions, just like yahoo answers)
2. Edit questions and its category at front end
3. Ability to see recent activities, i.e. which user answered which question, or asked which question (both at backend, and frontend a module should be there)
4. Funcitionality to log IPs so as to see if a user has not created multiple usernames for himself.
These are just few will suggest you few more.