Upvote = I agree with this, this is what I would have posted too if I had seen the comment earlier, this is extraordinarily funny or insightful and I want more people to see it
Downvote = I think this doesn't meaningfully contribute to the discussion at all, it would have been better if it hadn't been posted, others shouldn't have to read it
The vast majority of things doesn't fall into either of these categories, so I neither upvote nor downvote them; if I merely disagree with something, I write a counterargument but do not downvote.
Yes, but a lot of people "don't think this should be seen" simply because they disagree with it, no matter how much of a good-faith on-topic post it is. That was a main point.