What we have here is a pleasant, mostly polite community.  Before we start taking it apart, I think we need to acknowledge what we're doing right. And I don't think it's a coincidence that we currently have a culture where we can agree, give reasons for our disagreement, or say nothing at all.

Most misunderstandings in cyberspace arise from forgetting that you're talking to a real person and that normal courtesies apply. The 3 rating you suggest would involve abandoning the absolutely fundamental courtesy of saying WHY you disagree.  In normal conversation, we wouldn't just make a gesture or snap "No" and walk away, and a 3 rating would be just as aggressive here.