Actually, I think there's something to be said for downvoting. Not in the reddit "i disagree" sense, but in the slashdot/ meta filter "comments downvoted/flagged past a certain point will be hidden/deleted" sense. It would obviously take a lot of work to make that work within the media wiki software *and* the Wikimedia ethos, but it would probably save tons of grief and derails if the worst of the worst comments were limited by crowdsourced review.