... And much of the time, I think asking for a comment with a down vote is a good idea. Except, there are obvious trolls. Or someone else has already explained

... And much of the time, I think asking for a comment with a down vote is a good idea. Except, there are obvious trolls. Or someone else has already explained why the comment was inappropriate. It happens enough that the reason for the downvote seems—to me—facially clear ... so while I agree it's a good courtesy in most situations, I don't think enforcing it would help. Maybe a UI change could encourage this behavior (e.g., highlight "reply" on downvote), but this loses some of the clean look of HN ...