[mark-t]

I upvote things that I think are more important for other people to read -- that is, if I think it should be displayed higher on the page.

I downvote things that I don't think other people should bother reading -- that is, if they're uninsightful, irrelevant, or somehow distasteful.

The vast majority of things I just leave alone.

I think the word karma itself is partly to blame. It makes you think you're rating whether the comment is good or bad, but the effect of the up/down arrows is merely to sort comments based on relative quality/value for people who view the thread after you. For a while, I've toyed with the idea of suggesting a new interface that asks users to sort comments, rather than upvote or downvote, but I haven't thoroughly thought about it.

[jodrellblank]

I was thinking about two sets of up/down buttons.

"comment adds to/detracts from the thread" and "agree/disagree with the comment"

The idea being to have fewer controversial yet contributing comments downvoted to unreadability.