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Actually, Slashdot's system was the best, with Flamebait/Troll/Funny/Informative/Insightful options, with only a limited number of votes (they called it "moderation points") available on a given day (this was more complex, you didn't get mod points every day), and a meta-moderation system allowing to rate the way people voted on any given comment.

Since upvoting presumably promotes a post, and "noise/bullshit" presumably penalizes it, what would "I disagree" actually do?

You might say that HN already has this system, distinguishing between downvoting and "flagging".

It would presumably have a lower weight.

If "Agree" is +1, "Disagree" is -1, "Shit take/bad faith/troll" is -2, and "report" is threats of violence or spam.

I agree. Many times well written and relevant comments are down voted because many people have a different strong opinion on that while noise/meme comments climb to the top because they are funny and don't trigger people.