This metric has an attribute which is shared with average karma that I think is extremely bad, which is that it punishes users for making comments that get no upvotes. Or to put it another way, it punishes users for commenting on older articles (as those comments are significantly less likely to garner upvotes).
I agree that average karma is a very poor metric, and I found that my usage of HN changed drastically when pg started showing it more prominently. One optimizes for average karma by not commenting on stories that are older or less popular, as those are likely to produce a 1 or 2 rating due to lack of eyeballs. However such comments are often interesting and useful contributions, and it hurts HN to discourage them.
A possible solution is to scale by the number of people who look at the comment, although this might be difficult to do well. You could probably get better results by estimating a regression containing the following variables: age of post, score of post, number of comments, average score of comments in the thread, and the depth of the tree in which the comment was posted to get a good determination of how many points an average comment in that situation would get.
I think this brings up something that has bothered me a little bit about the comment system here. I would really like some kind of notification system when someone has replied to a post of mine because once a link falls off the front page, the conversation pretty much dies unless I randomly check my comments page.
That might be what the "notifo" field on your profile is for. They're some kind of notification service (YC2010) but I don't see any docs on how to use them on HN. (Not in the faq anyway and hnsearch just returns a bunch of articles about them making plugins and getting investors)
