Leaving an anonymous comment when voting.
Extending the "Please consider adding a comment..." reminder with a comment field bellow which allows to leave an anonymous comment reasoning the down-vote. This could help preventing revenge-down-votes and yet help the OP improving.
The field should be clarifying the optionality of this process, but as a motivation, half the down-vote-cost could be refunded as a reward. Or only half the down-vote loss for the OP is cast for unreasoned down-votes.