You don't think a downvote function could also be a good way to say "I disagree but have nothing more to add and I don't want to clutter the conversation"?

I suppose it could be argued that in that situation a comment explaining a downvote (that you agree with) would already exist, and that an upvote on that comment would serve a similar purpose to downvoting the comment it disagrees with.

There are plenty of times I disagree with a comment that don't deserve a response, however. Less frequent here on HN, but trolls trolling within the rules (to preclude moderation-based solutions) are an easy example of something that'd actually be better to downvote without a comment.

"You don't think a downvote function could also be a good way to say "I disagree but have nothing more to add and I don't want to clutter the conversation"?"

It absolutely could be, no sarcasm, but it would somehow have to be detached from "this comment is good/bad and should be promoted/demoted". "I agree/disagree" and "I think should be promoted/demoted" are certainly going to be correlated for as long as you are dealing with humans, but hard-wiring that correlation to 1.000 right in the design of the system is not going to produce good results. There is a reason all the successful upvote/downvote sites at least have a culture of explaining to people that the votes are not for agreement, even if it breaks down after a certain scale.

And I've never seen a successful two-dimensional moderating scale. (I've been keeping my eye out, and I'd invite anyone who could show me one to reply and tell me about it, please.) Slashdot came closest with what was about a 1.2 dimensional system where you could upvote/downvote with a set list of one-word reasons, but there was still no "-1 Insightful" or "+1 Contrarian Opinion You Need To Hear" or anything. (Capping at +5 also limited things.)

> You don't think a downvote function could also be a good way to say "I disagree but have nothing more to add and I don't want to clutter the conversation"?

Can you give an example in what situation this could be useful? When you disagree, there are many ways to disagree. If you just want to drive-by disagree, without making a proper argument, then perhaps it would be better (for everybody) if you just didn't engage at all, and let other people deal with the situation.

Think about who you're doing the moderation for: other people. It's your job, as a moderator, to explain and educate well-intentioned people that you disagree with why do you disagree with them, or what do you perceive to be wrong with their comments. Then, as an added bonus, community can judge also your moderation criteria, by moderating on your counterargument or explanation.

> There are plenty of times I disagree with a comment that don't deserve a response, however.

For obvious trolls, there should be flagging and eventual deletion.

Update: Actually, it could be useful to "disagree but have nothing to add" when you already agree with some of the responses. So, IMHO, the rule should be, you can downvote a comment only if you either (a) added a response or (b) upvoted an existing response.