The criteria for downvoting is simply "do you feel like downvoting". pg stated a while back that it is perfectly HN-acceptable to use the downvote as a mere statement of disagreement. I haven't noticed any forum staff mention otherwise.
I've seen downvotes on my comments even when they're neutral statements of fact ("dang is a moderator, recently announced" got a downvote from some HN denizen). Most people seem to follow the criterion you lay out though. Ultimately if a good comment gets a downmod that makes it less visible (why, oh why, does only one single downmod do this?) someone else usually comes along and 'corrects' it back to normal visibility.
Some original guidelines suggest it was ok to down vote for disagreement but I'm not sure "perfectly acceptable" would be the best way to put it. And more recently I believe he was even less encouraging of the practice. My own view is that down voting should never be used for simple disagreement (as long as argument is decent and good faith).
I'd agree with that. Comments I disagree with that are well-thought-out often add a lot to a discussion, or at least to my understanding of what's going on and other people's points of view.
I do tend to downvote comments that contain obvious factually-incorrect information, though, even if the information seems to have been provided in good faith.
How often do you upvote comments like that? Those comments you disagree with, yet are well-thought-out, add a lot to the discussion or at least your understanding of what's going on and other people's points of view.
I don't think people do that enough and it leads to comment-bloat. "I'll make my point and then add in a platitude to draw in the upvotes, hopefully balancing out the downvotes I'll get for my opinion."
I'm probably slightly biased toward upvoting comments that I agree with, assuming similar good quality, but I tend to upvote comments I disagree with fairly often as well.
Well, 'perfectly acceptable' doesn't mean 'actively encouraged'. If you don't have a problem with something, it's acceptable to you, whether or not you endorse it.
I haven't seen a recent comment where pg spoke against the practise, but it'd be very easy to miss in the firehose of content (which is why I specified 'I haven't noticed'). Do you have a link to the relevant post?
I've seen downvotes on my comments even when they're neutral statements of fact ("dang is a moderator, recently announced" got a downvote from some HN denizen). Most people seem to follow the criterion you lay out though. Ultimately if a good comment gets a downmod that makes it less visible (why, oh why, does only one single downmod do this?) someone else usually comes along and 'corrects' it back to normal visibility.