I upvote comments that--in my totally subjective opinion--deserve it. If I can tell that the commenter actually read the piece, that is, and had a comment that added something.
Actually, the truth is that I upvote almost everything, because if you took the time to respond, if it's anything more than "good job", that's good enough for me.
Glad to hear that you upvote almost everything. Upvoted!
Well, I wondered if I was doing something wrong, so it's nice to have someone else say that's what he does.
Glad to hear I'm not alone!