This solves another problem that really bothers me: an upvote to a really insightful comment (like the parent) has the same weight as an upvote to a comment that's merely useful, or, worse, that merely shares my feelings on some topic. Insight (in my opinion) should always count for more than anything else, and I think limiting upvotes will make it so -- because I think most people actually do care about it most.
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Another (even more dire, IMO) problem that might have a similar solution: comment threads are way too long. Only four or five months ago I could make a comment and I'd be absolutely certain that at least a few reasonable people would read it, and so it never felt like luck when my comments moved up; my good comments inevitably got upvotes, and my mediocre comments stayed where they were. Now comments get lost in the wash. I'm tacking this edit onto this comment partially because it's tangentially relevant, but mostly because I don't have faith that anybody will ever see it otherwise.
That's a huge problem, and I suspect that it is in fact the fundamental cause of the degeneration of other social news sites. When people know that their contributions will be noted, they act like they're participating in a conversation. When the success of a comment depends on a metaphorical die roll, I think people adopt the mentality of, "I'll throw shit at the wall and see what sticks."
My solution would be to limit weekly "root" comments (comments that respond directly to a submission, not to other comments) with a formula similar to the one above (c ln(karma) + d), though with the caveat that comments that get more than a few upvotes get "refunded." But frankly I'd endure any kind of unreasonable limitation if it would get rid of the feeling that this community is gradually but inexorably turning into a mob.
I don't know if the ability to upvote should be related to a persons Karma score. That might make one group too powerful and reduce the egalitarian nature of the site. What if everyone was just limited to 10 upvotes per day? Do you really need more than that? The more you read HN, the more carefully you will have choose what to upvote.
In the same vein, addressing the problem of too many comments, the extent of right to comment could somehow be earned. Say an unproven user can only comment in particular circumstances, maybe on threads that aren't already heavily commented on?
Let new users prove themselves in the lesser leagues, so to speak, before allowing them free reign on the site proper.