If you consider that a post needs a downvote, at least have the "balls" to assume it, give a reason, take responsibility for it, have the decency to leave a comment with a constructive reason. That's how you add value , by giving the author feedback, that's how you help him and the community as well.
Great idea! This should be mandatory and highlighted in the reply. If they can't leave a decent explanation, it makes the downvoter look like a jerk.
Thank you!