I don't know what the numbers are, but my first thought would be "growth". Online communities tend to be awesome until they hit a certain level of success, and

I don't know what the numbers are, but my first thought would be "growth". Online communities tend to be awesome until they hit a certain level of success, and then they represent a large-enough audience that people start performing instead of participating. Site admins and community norms can speed or slow that process, but AFAICT it's inevitable.