Interestingly, I was under the impression that on HN down voting was supposed to be for when comments are (provably) erroneous, disrespectful, harmful or just plain don't really contribute constructively, but not when I disagree about something that isn't definitive. Looking at the guidelines, they actually say little about what down votes are supposed to be used for. I wonder how I got that impression? Was it learned through the culture here, or is it wishful thinking on my part, or a bit of both?

In any case, I'm much more hesitant to down vote here just because I disagree with an opinion than on Reddit, and I think a lot of people do the same. To me, that higher standard for what's both considered a good comment and what's acceptable are what makes HN more useful for interesting discussion. There are probably 5-8 aspects of comments that could be voted on, but what we see with these singular rating systems is that the community forms around what is acceptable and what is not, and people generally fall in line with the community norms (both in submissions and in their own rating of others). Reddit is interesting in that I've noticed that this community norm is somewhat malleable based on the subreddit. /r/gifs and /r/programming have a distinctly different feel in the comments.