I don't vote if a review just isn't interesting enough to read, or if I find that it's poor quality, but a reviewer's first or second review. As others have said, I am not going to fudge ratings, but I'm not going to mess up their rank forever.
I rate almost every review I read.
Except in cases when: I didn't thoroughly read it, I thought it wasn't particularly useful, but can't offer feedback because way too much was missing and some of the other cases mentioned.
I don't like to be mean either, but if I do leave lower feedback, I usually ask a question pertaining to the key thing that I thought was missing.
I also leave feedback about things I really liked that the person added, especially when it is something people don't normally think of or it was really helpful.