I often hear people advocate a policy of only downvoting offensive or trolling comments on news.YC. I think it's a bad idea.

My goal in voting is always to improve the experience for the next reader. That means that I vote up what I think is the most interesting or important information, and vote down distractions and annoyances of all kinds: not just trolling, but also poorly thought out arguments, boring observations, rambling, point-missing, and anything else that consumes space without providing value.

You can look at that as being mean to people who posted comments that were merely not top-notch, but I think of it as being kind to the probably much larger number of people who are skimming the comments looking for something worthwhile.

The problem with your approach is you are judging what is valuable for the next reader. Your judgment might be good most of the time but it does not imply an all-round more efficient system. I'm sure digg and reddit were conceived in this spirit. And when you have a large number of people thinking like this, you automatically adjust the majority of stories to the majority view.

This is quite a debatable flaw in your argument. Worth a downmod? Hardly. I am pointing out the obvious to some (or many), but to others, and those who share your viewpoint, this might be a blind spot, or simply preposterous.

I think a community like this (HN, that is) is better off just getting into the habit of good discussion, such that it becomes implicit in the culture (which kind of is already). Just make sure the SNR is naturally high. That's quite a feat.

That's pretty much my take too.

Although I would further add that I often take a particular pleasure from upvoting a comment that I disagree with, if it is suitably thought-provoking, challenging, and constructive.

I disagree with your second sentence, but I upvoted you for it.

I downvote for factual errors (such as 'Symbian is open source') because a high vote count implies that the comment is correct.

That would be a very unfortunate loss of information. A better scenario is if you reply with the correct statement, and try to make it upmodded more than its parent.

If you sink the incorrect comment that previously had a high score, those who walked away with the misinformation have a higher chance of having their misconceptions persist.