0: in particular, on HN the downvote button also seems to serve the purpose of "I don't think other people should see what you have to say" button due to HN's passive-aggressive greying out of downvoted comments.
Requiring a comment is probably a bit much, but I do think two separate downvote buttons would be helpful to the commenter, even if only they could see this feedback. Maybe one link labeled with "disagree" and one with "low quality" or something. UI-wise this would have the added benefit of letting new users know that HN officially expects them to consider both options.
I don’t think it would be viable in the threads which touch on larger societal debates. A great example would be election topics after Trump started lying about election fraud and that message became something media outlets started pushing to millions of people on topics many people here are interested in like election systems or forensic analysis. That leads to waves of people repeating long-debunked claims ad nauseam and because they aren’t here to learn or even debate rationally, there’s not much point in filling up the thread with 200 comments saying “This is not true. See http…” over and over, and the volume means that the kind of people we’d most want to have involved in such a thread are going to get tired of it and move on.
One natural response is to say “no politics” but that’s really not possible given the involvement of IT in almost everything now and the areas where legislation is being proposed. The approach of having skilled people like dang moderate threads works well but it’s very expensive, so I think the community downvoting low-value posts is probably a necessary evil. It’d be tempting to have some way to say that someone isn’t contributing to a thread to boot them out but that seems hard to do without being too slow to matter or prone to brigading. Labeling might be worth trying, as much as a social cue to the voter as new information for the moderators.