downvotes are more mysterious than upvotes (though fixing that would require more like three or four categories)

What categories did you have in mind? I'm thinking "This post is harmful," "I disagree," and "I don't like you." Of these, I think only the first is really something you'd want a button for. It seems to me that anytime I would hit a disagree button, I could more usefully make a reply, or simply agree with someone else's reply. And "I don't like you" seems awfully mean, although I guess it might serve some purpose.