It's worth noting, though, that HN does have up and down votes for comments, but only users with moderately high karma can downvote.
This might not work well given relatively quick growth. A small number of people consistently downvoting bad comments may be outvoted by a huge mass of new users who from time to time upvote. Maybe everyone should be able to upvote AND downvote, but experienced users get double weight.
Personally, I think we need something different than minor adjustments to the voting weights/allocations. Slashdot-style moderation? Revamped guidelines? Hard to say what will work and what won't.