Then I remembered kuro5hin's decline and death. A self-moderated forum can transition from "temporary lull in activity" to "permanent death spiral" if a critical mass of trolls pounce on the lull, and it's not a pretty thing to witness. The "critical mass" doesn't have to be very large, either, since self-moderated forums generally weight their users' opinions proportionately to amount-of-available-free-time, which most trolls have in surplus.

Kuro5hin's $5 paywall for new accounts arguably deserves a lot of the blame for that. (My fading memory of K5 is that the boundary between trolls & non-trolls was pretty permeable there, and it wasn't a huge problem until the paywall came down.)

[Edit, August 31: belatedly realized "the paywall came down" is ambiguous. I mean the paywall being imposed, like a curtain coming down.]