Attempts at moderation are usually futile because enough people
disagree with moderation decisions and begin to leave or argue with
moderators.

Moderators are the biggest problem with walled gardens. A good
moderator only blocks spam, not controversy. Alas, most moderators
censor opinions that counter their own, or are unpopular, despite the
fact that phbb has an "ignore" feature.

Good moderators are so rare, that I no longer find it worthwhile to
contribute to any private walled-garden forums. It's not worth it to
have a spam janitor combined if he's going to block interesting posts
at the same time.