For some reason I would feel much better imposing a standard cost on commenting (e.g. -2 karma) that can be easily balanced by being marginally useful. This would better disincentivise both spamming and comments that people didn't expect to be worth very much insight, and still allow people to upvote good-but-not-promotion-worthy comments without artificially inflating that user's karma. This however would skew commenters towards fewer, longer, more premeditated replies. I don't know if we want this.