Also, (sorry no drawing yet, just got home I'll do it now :p),
User contests boost the morale of the forum/site, and the interactivity. It steps the community a bit out of just discussing elements of the games, and getting the community to laugh, work together and pull out a creativity site.
One that popped into my head, which is pretty simple, have a contest for the best fake GTAIV Box Art. Now, I know real screens are out for the game now, but remember that most GTA games use artwork, that they don't previously release. Give marks for the most creative box, including even printing it out and slipping it into an xbox360 case, for kicks!
Prizes may be as simple as a special title on the forums (similar to the LEGEND one), or a spot on a contest winners page.