What about this: to to downvote, you need to also specify a keyword as to why you are downvoting. It's a text field (not a pull down). Whatever you enter, will not be visible to anyone except to the person who wrote the comment you are downvoting.
Adds just enough friction to reduce downvotes a bit, provides information to the person being downvoted (no more "why all the hate?" comments), and doesn't pollute the experience for everyone else, since those are not visible by most users.
Adds just enough friction to reduce downvotes a bit, provides information to the person being downvoted (no more "why all the hate?" comments), and doesn't pollute the experience for everyone else, since those are not visible by most users.