All really good forums, in my experience, have attentive and firm moderators. (HN for example.) A few years ago, I moderated a developer forum (not on Reddit) and by posting clear rules and enforcing them consistently without being a jerk about it, the forum was indeed evolved into a useful place for conversations. But it took constant attention; and I think most Reddit moderators don't have the inclination to do that work day in and day out for the sake of the community--and so the quality progressively deteriorates.