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.