I suspect the average karma preference has a negative effect on quality, as you've pointed out. There is incentive for the best commenters to only comment occasionally, when they'll get a lot of karma for it.
Digg kept piling on stuff like that, making some users more valuable than others according to the algorithms, and I think it hurt rather than helped. I'm not sure it hurts on HN, but it doesn't seem to solve the core problem, either.
I think it does come down to a social problem being tackled by technical solutions. It may be that the only way to make this problem go away is to empower moderators (of which HN has more than a few) to simply remove the most negative stuff before it has a chance to alter the tenor of a thread. Hostility breeds more hostility and it spirals ever upward, until the conversants are more frustrated than talkative.
Digg kept piling on stuff like that, making some users more valuable than others according to the algorithms, and I think it hurt rather than helped. I'm not sure it hurts on HN, but it doesn't seem to solve the core problem, either.
I think it does come down to a social problem being tackled by technical solutions. It may be that the only way to make this problem go away is to empower moderators (of which HN has more than a few) to simply remove the most negative stuff before it has a chance to alter the tenor of a thread. Hostility breeds more hostility and it spirals ever upward, until the conversants are more frustrated than talkative.