Reddit's solution (and I'm not saying it's perfect, but it seems pretty effective) appears to be that the community can push a simple up or down arrow next to each comment. Supposedly, admins don't normally intervene.
Usually this arrow system just fulfills the role of applause/boos -- I initially suspected this was a bad idea, but maybe it's a good feedback system; you get an idea of what people want to read. Many won't down-arrow you if you write a well-reasoned comment which they strongly disagree with.
With enough down-arrows, the comment is mostly hidden from view. (But a clickable sliver of it remains, if you wish to look.)
In my experience, the system works fine, with relatively few problems. There are of course holes, but I think it scales better than central moderation, especially where there's a scary warning as you walk in, with the feeling you're entering hostile territory.
Tayssir