Your proposal sounds similar to the flagging mechanism on many online forums, where the person who flags the comment is asked to specify a reason so it can be t

Your proposal sounds similar to the flagging mechanism on many online forums, where the person who flags the comment is asked to specify a reason so it can be triaged by a moderator. In the case of downvoting, it doesn't rise to the level of needing to be triaged by a mod, but I suppose you could argue that by giving the commenter a signal about why you downvoted, you're allowing them to self-triage (e.g. update the post to address the issue, or at least try to do better next time). Being able to choose from a short list of predefined downvote reasons -- even if it were between "This breaks forum rules" versus "I simply disagree" -- would be useful.