I know I've left a lot more comments because of kudos, than I would have done otherwise.

I've had a couple of negative run-ins with some authors based on reviews I've left, (And I'm a squee only reviewer, pretty much.) so in order for me to leave a comment I need to be in a very positive mood, leaving no feedback tends to make me feel more negative about myself.

I'm very much of the belief that comments are much more about the people who left them, than the actual story.