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.)
You can't downvote until you have enough karma, and a comment cannot be downvoted after it's been around for more than 24 hours. This is to prevent someone from vindictively going through a person's "threads" and downvoting all of their comments in an effort to kill their karma.
You can never downvote article submissions; they can only be upvoted. Downvoting sumbmissions has merit in some cases (removing spam), but in practice it has been used on other sites to kill everyone's submissions except your own, rapidly leading to a downmodding war where the winner is whoever is more committed to their karma score, instead of the one who is submitting the best articles.
