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.
Too much information can be ignored, too little information is sometimes annoying. I'd always welcome your reason for explaining your downvote, especially if it seems legitimate to me.
If we were going to get highly technical, a somewhat interesting thing to do would be to allow a double click to differentiate your downvote, and divide it into several "slider bars." People who didn't differentiate their downvotges would be listed as "general downvote" Those who did differentiate would be listed as a "specific reason downvote." ...
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).