If I were to rephrase the question on this thread, it seems to me that it could also be stated as "how do you keep HN comments from turning into Slashdot comments"? I don't say that in jest - I used to read Slashdot, but after a while I got really sick of 1) the vitriol and 2) the inanity of the comments that were on the first page. Granted, some folks had really interesting things to say, but truly funny/insightful comments seem to be a rare commodity.
But look at it in a positive light - the comments on HN could never be classified in the same - or even near the same - bucket that comments on sites like YouTube and Yahoo! News.