It seems to me software designed to facilitate discussion shouldn’t have a downvote buttton. There should be a UI for marking comments as inappropriate, but it should require a second step saying why. Perhaps one of the reasons should even be “I disagree”, but that option should have no effect.
It’s not impossible to abuse of course, but it nudges people in the right direction. Those UI nudges can be pretty effective.
We have discovered a revolutionary shape: the rectangle!