Quote Originally Posted by Dododecapod
Even if you don't want to make a big, formal review (I tend to write screeds, but I have some formal reviewing training) you can always say "I liked x aspect" or "I didn't like x aspect". At least the writer then knows you read it and cared enough to acknowledge his work.
We as authors also have a responsibilty to reply to our reviews and fan-mails. I know I have written private notes to more than one author and never heard jack back. That tends to discourage me as a reviewer/fan-mailer.

And to echo what others have said: I don't read most of the stories posted here, and don't review most that I do read. I generally only review tales that turn me on. It is not easy to disambiguate my dislike of a story for reasons of content from my dislike of a story because it is poorly written, so I generally don't bother. But that's just me.