I disagree. ;) Specifically because knowing who disagrees with me gives me way more evidence than just knowing that someone disagrees with me. Practically speaking I do not consider downvotes evidence that I am wrong (they are generally just evidence that people dislike what I say because it sounds pretentious or because it pattern matches to something that could be wrong), whereas I would consider a simple "I disagree" comment from e.g. Nick Tarleton some evidence that I was wrong and should spend effort finding out why. (This isn't mostly because Nick is a good thinker (though he is) but that I think it's a lot less likely he'll uncharitably misinterpret what I'm trying to say, whereas an "I disagree" comment from Vladimir Nesov is a lot less evidence that I'm wrong even though he is also an excellent thinker.)

Obviously "I disagree" plus a short reason is better and normally not much more difficult, but this would also be a lot easier to do in a community where just "I disagree" was acceptable.