Having thought about downvoting your post (but not, -2 seemed low enough): the problem is not obviously one worth solving. Your idea does not obviously solve the problem (if your comment had lots of downvotes with the reason given as "troll", would you feel more informed?). There are obvious downsides to your idea, as discussed by the other replies, that you should have addressed. Any benefit from your idea, when it has been implemented on other sites (see e.g. Slashdot) is dubious.
Actually, "troll" does offer some information. It means the downvote was because the comment was perceived as an effort to stir up pointless conflict, rather than for it being wrong, poorly proofread, redundant, or a multitude of other possible reasons.
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).