Lowtax (he runs somethingawful.com) gave a talk about web communities at UIUC a few years ago and he basically said without good moderation large sites will just degenerate into crap. The talk(http://www.acm.uiuc.edu/conference/2005/video/UIUC-ACM-RP05-...) takes 15 minutes or so to get into anything serious, but I think there are some good lessons in there.