What do you guys think about taking the Slashdot approach, with different kinds of votes for different reasons? On Slashdot you can (up-)vote a comment as insightful, informative, interesting and funny. In the case of downvotes, splitting them out by "I disagree", "not useful" vs. "offensive" vs. "looks like a troll" might be helpful especially to raise flags for moderation. If then we could disable the display of these categories as decided by moderators that might make reading comment threads less annoying. (I don't mind if you also allow changing what's displayed based on straight votes, but that's easily gamed.)

Edit: After the discussion below, I have a suggestion for another type of downvote: "Cannot comprehend argument." (I smile as I write this, but I'm quite serious.)