I havent put a lot of thought into an ideal solution to this problem. But heres the best I have so far. Change the definition of karma to capture notability which would be a monotonically increasing number. So when a person mark a comment as notable, it increases your karma score by 1. Seperately have a agree / disagree score per comment whose lifetime is limited to a particular thread. Within a thread a reader can use the agree / disagree scores of comment to selectively read both sides of the debate.