Of particular interest to me are Wikipedia and StackOverflow, where the pedantic rule-following, rule-enforcing insider group eventually squeezes all joy and then all productive life from the the system.
Actually, I have little hope for Wikipedia's solving the problem, due to their governance model and political structure, but StackOverflow has economic incentives to solve the problem, so I follow them with interest.
Reddit seems to remain vibrant, albeit often a wretched hive of scum and villainy depending on subreddit…
Add Tildes to the list of projects I'm watching with keen interest. I believe the question of how to keep a massive online community healthy is actually a key problem of our times.