I Quit Hacker News

    In an ideal community people would up-vote arguments for adding
    value to the conversation and down-vote only for detracting.

I agree in general with this point--I'd much rather have comment ranking based on comment value than how many people agree with it. However, I think a big part of the problem is that there's really no other way to 'agree' with a comment. Commenting "I agree" or something similar is (rightly) frowned upon, but the only real replacement right now is an upvote (or, if they disagree, a downvote), and people like being able to indicate their support of things.

The obvious (although maybe not best) solution would be to add another axis for comment voting that doesn't affect post ranking (or, at least, affects it much less than the current one) that tracks agreement/disagreement. I can see a couple of downsides to this, the biggest of which is that it adds a large new element to a very simple commenting system.