"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.)
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.)