Why downvote? I honestly do not get it. 
jlees: It's just behavioural reinforcement. Upvote the positive but on the few occasions someone acts out of line with the spirit of the community, there's got to be a way to deal with them. Imagine a troll running rampant on HN with no downvoting; it'd be hard to get them to stop, since there is no consequence for their actions.As it is, behaviour that acts out of the norm - lame jokes, empty-but-offensive remarks and entirely pointless trivia - all tends to be rewarded negatively, reinforcing the community values nicely. If you get downvoted, it's usually simple enough to figure out why - and if it really is a disagreement of opinion then the downvoters are doing it wrong, or there's also something about the way you express your opinion that is repellant.