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"If you have low karma, you can't downvote people"This is a good thing. It makes it difficult to create a downvote-bot-brigade. It means that only people who have participated in building the community up to a certain degree have the right to act as gatekeepers.
> "you can't post very much"
I'm not aware of any such restriction, except for very-very new accounts.
> "it's absolutely to your advantage ... to self-censor"
In a sense, this is by design. If downvotes don't help people learn what sort of comment is and isn't welcome here, then we'd be overrun with cat pictures. Downvotes are supposed to teach you "this doesn't belong" just like upvotes are supposed to teach you "this is an appreciated comment".
I think it's even OK that occasionally a decent comment will pick up a couple of downvotes because it's slightly disagreeable, poorly written, or just that someone clicked the wrong button on accident -- a few random points of negative won't hurt someone whose contributions are generally positive.
Where it becomes a problem is when well-thought-out, civil, reasonable comments get heavily downvoted. When someone with a contrary opinion can't make any positive progress. I try to make it a point to toss upvotes on comments like that, but perhaps more could be done.
One thought: even if we don't require a comment box for all downvotes, perhaps requiring a comment box for downvoting something beyond a certain level of negative (like, beyond -1) would keep people from piling on mindlessly or needlessly.