The scarcity of mod points helps a lot with the type of posts that get upvoted. I mean, I just saw a Digg article (well, picture) where the most dugg comment was "Ha Ha! That was pretty hilarious...and true.", in response to some 4chan meme image. The good thing about limited upvoting is that people will usually only upvote comments that are either particularly witty or of substance, not thoughtless drivel that inspires no conversation at all.