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HN doesn't really allow downvotes though. At least not unless you've reached a million karma points (from my understanding).

I became able to downvote after reaching 500 karma. The minimum requirement might have changed to 1000 since then, I forget.

Even after reaching that karma amount, you are only able to downvote comments within a day of their posting. After a day, only upvotes are possible.

It does. I don't know what the threshold is, but it's not too high. I believe it's 500, but I might be wrong.