Comment and explain your downvote, you'll likely be downvoted for doing so, or be accused of trying to be a mod, or waste your time explaining yourself to someone who wasn't even interested enough to post a substantial comment in the first place. The main reason I downvote is "low quality comment" which is subjective and I don't want to enter into a debate about arguing that it is against the author arguing that it isn't; it's a
voting system, I vote, you vote, we all vote, and a consensus comes from that. I don't want it to be "my judgement is better than everyone elses", I want it to be my vote among many.
(Or rather, I do want it to be "my judgement is better than everyone elses, and I want to be able to delete other people's comments", and I recognise that's bad, and that a vote system limits the maximum damage I can do to one vote per comment).