Most of the time I confess that I don't upvote the topics I comment on. I just forget to do it, mea maxima culpa.

Maybe the ranking algorithm could be changed so commenting on a topic is counted as an up-vote. That would have the bonus benefit of riding us of the "this doesn't belong in HN" complaint comments.

(Disclosure: I didn't see the Obama discussion, wouldn't have opened it had I seen it.)