I care because it's a measure of whether what I say is agreeable or disagreeable. It's not about fitting in or following the herd, but if I say something that loses me karma I want to understand why. Maybe I'll end up with the same opinion anyway or maybe it teaches me to think more before I talk. Mostly, karma helps me reflect on my opinions and question those opinions.
The only thing I would do with HN is to _force people to comment when they downvote_
If you agree with me, then an upvote with no commentary is fine. If you downvote me, tell me why. Maybe I could learn something, maybe I get the chance to explain myself better, maybe an open disagreement on a point enriches the community.
Except, there are obvious trolls. Or someone else has already explained why the comment was inappropriate.
It happens enough that the reason for the downvote seems—to me—facially clear ... so while I agree it's a good courtesy in most situations, I don't think enforcing it would help. Maybe a UI change could encourage this behavior (e.g., highlight "reply" on downvote), but this loses some of the clean look of HN ...
How do you prevent a: 'Downvoted for being moronic->No, you are' kind of thread?
I guess if comments were forced on downvotes, let's say "I downvoted you for being a moron" than instead of leaving your own version of "you're a moron" you could add your weight to the first downvote-comment.
So like downvoting with a comment, but kind of adding your weighting to someone elses downvote for the same reason.
Edit: I want to clarify that I am in favour of deducting karma for new submissions. The comment system is already fairly balanced thanks to the option to downvote. What I am suggesting is essentially a mirror image of the parent article.
i.e. people who post 'crap' and get continually downvoted will eventually be silenced.