Karma/Comment is not a very good metric as well. As someone that posted a big post that stayed in the Front Page for long can amass a lot of karma in little time. Giving them a very high Karma/Comment. You would have to remove the outliers.
Karma/Comment is a poor metric, I think, because it encourages the strike attack comment style.
If I post something and people post counterpoints or questions or concerns, I will respond. But the truth is that from a pure karma average perspective, by far the best action would be to simply leave the threads hanging, the single post floating at the top of karma heaven. That works great from a pure karma perspective, but it subdues conversation because every point is a single attack.
It also encourages the stealthy comment editor. While Paul mentions this, it is something that happens regularly -- people make a comment, someone responds unfavourably, and the original person subtly edits their comment to make them look unfairly attacked. This is a specific problem on HN given that quoting is generally discouraged. So everyone piles on the downvotes and upvotes, respectively, to right this seeming injustice.
Karma on HN is easily gamed, as it is elsewhere. It is unfortunate when we care too much about it because it leads to completely artificial conversations.
