I believe downvoting is only available to users with high karma. This makes me believe that downvoting is meant to fight misbehavior. Unfortunately there is no way to report misbehavior of downvoting. If you already have high karma you have a conflict of interest in reporting downvoting misbehavior.

Actually, it seems that downvoting for disagreement is a ok behaviour:

pg: http://news.ycombinator.com/item?id=117171

I will stop using downvotes for disagreement then.

(Actually I think it is the first time I use this downvote feature on HN so the damage is fairly limited so far).