"without having a measure of the posters' relative popularity" I would add also that I'm sure there are people that don't realize that a high karma score can c

"without having a measure of the posters' relative popularity" I would add also that I'm sure there are people that don't realize that a high karma score can come not only from comments such as you describe but additionally simply because someone was the early poster of a story that got voted up. So in a certain karma range we don't know how many points are from that vs. from agreement with a comment. (My guess though is that as karma increases most of it comes from comments and activity that is not related to posting a popular link).