I downvoted your comment because you seem to be trying to solve the problem that your first post got downvoted, not the problem posed by the OP. Do you have an argument why requiring explanations for downvoting will reduce trolling and increase the signal to noise ratio on LW? Note that the founder of LW has argued for being more downvote-happy.
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).