Should we force adding a comment with every down-vote?
Anonymous down voting is not constructive, and the individuals who choose to hide behind the click, most often never up vote a revised question or answer.
I think if you anonymously down vote a question and it begins to receive up votes, each up vote should cost you rep, because in that case, your down vote is standing against the reputation of your peers. Now that's community moderation!
I am requesting this as a StackExchange feature of community moderation. There are (known to me) individuals who's job performance ratings depend on their reputation. The lack of such a feature invites harassment, embarrassment, and defamation of character.
This should not go ignored IMHO. If this isn't something that can be implemented then the act of down voting should pop-up a comment box asking why the question/answer was down voted so that the proper course of action can be taken by the author, and anonymous down voting is rendered as an unacceptable practice.
Many community members have already taken to the (+1/-1) notation in their comments about voting. I find this to be acceptable, positively constructive, and a feature worth implementing.