Leaving an anonymous comment when voting.
Note: this proposal is the similar to a low-voted response to Encouraging people to explain downvotes, added (by someone else) after the question was resolved.
The problem: downvotes without comments are annoying to responders, and ultimately do not provide any value to the poster or the SO community as a whole. Unless a response goes negative, there is no indication that the answer may be incorrect, and the responder does not learn why he/she is incorrect.
However, members of the SO community do not wish to fear retaliation for their downvotes, so do not want to be forced to provide a comment that explains their vote. Looking at other questions where this issue has been raised, it also appears that some people simply don't want to bother giving a reason for their downvote (although this attitude doesn't seem very community-oriented).
I propose that clicking the downvote arrow will require that the user enter a comment, then attaches that comment to the response/question without a username (or, if the DB schema requires a user, with a dummy username).
Yes, it's possible to game the system by typing "asdasdasdasdasd" (I think that's 15 characters). However, doing so will make clear to the downvoter that s/he is a borderline sociopath. More important, will signal to the responser -- and the community as a whole -- that the downvote was given without a lot of thought, and is therefore suspect.