Getting users is allways the most important point for any site.
I allready commented about the : Send question or thread to friend function
but of big importance could be as well the invite function. Getting points for that would be obvious, but earning a percentage of points generated by the invited user on the long term, lets say for a year 10 %, would be new and some users would permanently try to activate friends.
The badges !? or similar gratifications alongside with user classes from beginner to expert would make it even more interesting.
Any opinion is welcome.