Several of the high-quality forums* I read explicitly (ha) do not have formal rules; the rationale being that having them written down enables the antisocial behavior of doing the worst thing that's still within the rules. However, these forums also have attentive and active moderators (as opposed to silent-except-when-things-go-seriously-wrong moderators) who speak up to discourage bad patterns early, which is not the case for Less Wrong and probably can't be made the case.

* forums in the general sense, not in the genre-of-web-site sense.