I don't know what the effect was historically but right now if there's too many downvotes a comment gets greyed out. Furthermore you have to have over several hundred karma to be able to do it in the first place. In other words, the behaviour isn't symmetrical with upvoting, so I find it very uncomfortable, even for myself, to downvote merely for disagreement (i.e. the opposite of an upvote).

Same here. I reserve downvoting for unhelpful comments.

One litmus test for unhelpful commenting is, that the word 'you' is used. I try to keep it on topic, and the topic is never the other commenters.

There's gonna be tiny exceptions, but this is an excellent guideline for comments that I will try to follow in future

Well, I disagree with the guidelines. I beleive hiding text from others, just because you don't agree with it, is censorship.

It discourages open discussion.

I'm with you. History is filled with ideas, words that were originally laughed at/down-voted.

I have never felt the need to down-vote. I hope I never turn into that guy.

I inderstand moderation. I understand the up-vote. Why make a post unreadable? If needed--put a minus sign on the post, but don't make it difficult for people to read. These posts go onto Google forever. It's just un-American. That's been my only gripe with this site.

People quoting the guidelines won't change a thing. There'll always be threads like this one if downvoting is clearly abused.

And hardly anyone cares about pg's opinion, especially in a thread about FOSS, to which he has contributed fairly little.