I've wondered about that too - what motivates people to vote something up or down? Is voting up more a measure of interest, agreement, surprise, appreciation...? Would you vote down because you don't agree with a comment or because you think it's irrelevant, self-serving, unoriginal, or poorly-stated? (I didn't see anything I thought worthy of downvote on the parent comment either; I vote it up as thought-provoking even though I didn't quite get the point of the thing he linked to.)