What we've seen happen with self-regulating communities, both real and virtual, is that they go through a brief initial period during which their performance improves - a kind of honeymoon period, when people are on their best behavior and rascals are quickly exposed and put to rout - but then, at some point, their performance turns downward. They begin, naturally, to decay.As I said before, there is a repeating pattern with community sites. They start with great buzz and joy from an enthusiastic group of early adopters, then fill with crud and crap as they attract a wider, less idealistic, more mainstream audience.
Leave them alone long enough, and they're far more likely to collapse than to reach perfection.
When there is an incentive to abuse a system, people will abuse it. From the start, community systems need to be designed to filter out crap and spam. Without intervention, without tools to surface the good and bury the bad, a sea of noise will drown out any wisdom that could have been found.
Please see also my older post, "Getting the crap out of user-generated content", where I quoted Xeni Jardin as saying, "Openness has its downside: When you invite the whole world to your party, inevitably someone pees in the beer."
Please see also my other posts, "Buying votes on Digg", "Digg struggles with spam", and "Summing collective ignorance".