How about a reward system where user can redeem their points. I think this will motivate the visitors of that site to become regular users. And also encourage them to earn more points by asking more questions, answering more questions ..... and thus speed up the growth of your site dramatically.
So, what is the redemption? Redemption is a system where user can use/consume their points to whatever you offer.
For example: 50 Points = $10 prepaid mobile card / $10 worth free ebooks
100 Points = $20 prepaid mobile card / $25 worth free ebooks / Gift voucher, etc.
A referral system into this will add extra flavour. For example, for 1 point gained by referred user will add .25 (1/4 of points earn by referred user) point to referrar.
Hope anyone will attempt to implement this plugin.