Generally either the users or the admins are unhappy with the moderators of any given subreddit. They're on possible way around it but not a particularly good one and it's less likely to scale because there's not the same "ownership of a community" feeling you can engender in Twitter where there's not really an equivalent to subreddits to give volunteer mods control over.

And reddit has not solved the problem.

To many, reddit is just as unpleasant, and in some cases worse than twitter.