It seems to me that the (arguably) best discussion sites are those governed by benign dictatorship: firm moderation and clear guidelines. Yes, there are edge cases where judgement must be made but clear guidelines make this easier.

Whenever I think of a good example of a community that has managed to stay pretty high-quality for some time, I can't help but think of Matt Haughey's Metafilter. I'm sure the nominal charge for membership helps, but the (paid) moderators are very good.