You need 20 karma to down vote and each vote costs one karma point.
Actually I like the idea - it would prevent (or at least raise the cost of) karma-bombing. I think the threshold should be higher than 20 though - maybe 50 or 100. It would probably be best to introduce it at a high minimum and then lower it after observing activity, like pg said he's doing with the polls.
What does a down vote mean? "Do not want"? I'd personally be tempted to down-vote polls.
Edit: better yet, they should be revealed unconditionally.
I'd gladly give up karma points to vote for bozo articles. The value of this site to me is greater than any point system.
-weight it versus upvotes
-weight it based on user history
-weight it based on user's propensity to downvote
-weight it based on the number of distinct "user clusters" it offends
(If one group always votes for butter-side up stories, and another always votes for butter-side down stories, and a story annoys both of them, chances improve that the story is worth voting down.)
Another interesting metric might be contentiousness. If a topic generates an outlying number of downvotes from trustworthy users, and an insignificant number of upvotes (or many upvotes from unpopular users), its participants can be labeled as positive or negative forces. Someone who posts articles that generate a lot of downvoting in the comments might not be picking the best topics. There's probably a nice linear algebra way to generalize this in the same vein as PageRank.
A closed-source, nearly-null downarrow would at least give pessimistic users the feeling that their voice was being heard. (Heuristics is one area where it makes sense to keep things closed-source. Changes should be explained, but not necessarily in exacting detail.)
I left Reddit because that's what happened with every story. You submit something and it gets burried within 10 seconds. If it was 10 minutes, sure some people red it but 10 seconds is barely time to read a title.
I am downvoted >:) I can only upvote nodes, so I am not sure how I was downvoted.