> I personally think of "disagreement" as a continuum, where one end is pretty close to my own view, and the other is absolutely abhorrent or incoherent.
Yeah, I like this perspective but I think there's two axes we can separate a little. One axis is disagreement. (I agree with you <-> I disagree with you). the other axis is effort / coherence. (I'm going to invest significant effort to clearly articulate my point of view <-> I'm going to make a flippant off-handed comment).
Disagreement alone isn't the problem - I've had some lovely conversations on HN with people I disagree with. Eg, a couple weeks ago I had this interesting exchange[1] talking about RMS on the FSF board. But the conversation worked because everyone was careful and articulate. When someone says something I disagree with, but they say it in a thoughtful, well articulated way, I learn something new.
There's probably more factors at play, but as a rule of thumb I suppose downvotes are in the low effort + strong disagree quadrant. In that zone I feel like there's no implied intent to even talk it through. And absent conversation, why are we even here?
[1] https://news.ycombinator.com/item?id=26611767