I'm imagining an optional checklist offered if you downvote, with the list possibly including troll, poor spelling/grammar, false, redundant....
For that matter, if an optional checklist for downvotes makes sense, then perhaps there should also be one for upvotes: sensible, informative, funny, caused an update....
I'm imagining a little chart appearing if your cursor is on a karma number, the way the proportions of stars do for amazon reviews.
I don't know how much trouble this would be to program-- I'm just floating the idea.
http://xkcd.com/810/
Of course, this is a straightforward problem to fix in the mechanism design: Just make responses to downvoted comments start at -5 karma, instead of having a direct penalty, as suggested elsewhere. I think that suggestion was for unrelated reasons, but it also fixes this little loophole.