I generally upvote when I see a comment that makes a good point I think is underrated. You could argue that this is a kind of agreement. But, in my view, agreement alone is not the only criteria. Stating obvious truths isn’t really worth anyone’s time, even if they are agreeable. I will also upvote posts that changed my mind or are close to doing so, or impress me (insightful, or funny).

I down vote spam and posts that misrepresent a position or argument (straw man).

I will not vote at all for most posts.