(+1) I rarely downvote, but from now on, I will accompany any downvote with a reply stating "-1: reason for downvote."
I will downvote every such comment because I oppose this being used as a general policy. I don't want to see the spam and sometimes it just isn't useful to criticize explicitly. Even when downvoting is accompanied by such an criticism it is sometimes better to just speak directly rather than dragging in talk about your downvotes as part of the conversation.
This seems like a reasonable approach. The reason for the downvote could force a defamatory statement, which I prefer to avoid. Otherwise, you are right that dragging in a downvote mention doesn't add anything to just saying what you want to say. Thanks for the comment, by the way.
I was thinking that upon downvoting, maybe an option (not a requirement) should be given to state a reason why. Then I realized that there is no need to program such a thing; this option exists already.
The recent implementation of a -5 karma penalty for replying to comments that are at -3 or below has clearly met with some disagreement and controversy. See http://lesswrong.com/r/discussion/lw/eb9/meta_karma_for_last_30_days/7aon . However, at the same time, it seems that Eliezer's observation that trolling and related problems have over time gotten worse here may be correct. It may be that this an inevitable consequence of growth, but it may be that it can be handled or reduced with some solution or set of solutions. I'm starting this discussion thread for people to propose possible solutions. To minimize anchoring bias and related problems, I'm not going to include my ideas in this header but in a comment below. People should think about the problem before reading proposed solutions (again to minimize anchoring issues).