I tend to think about this subject from time to time. The problem that I see, at least with user-driven sites, is the difficulty in the beginning. In order to get users, one should already have a decent amount of content on the site to begin with. However, due to the fact that the content is provided by the users, where does this initial content come from? Does it all just start with a small handful of users? I feel that even when creating a buzz, there should be some content on the site. This would give the users a reason to return.