If someone has asked for negative comments and there is an obvious flaw, I do as Sam recommends and point it out gently.. even then, I've had an inbox full of flames.

I am also careful with the ratings... people expect 5 stars and I have been sent emails for giving fewer than 5. The way I see it is 5 is the best and there are some artists who, IMHO, deserve 5 stars. But they are few and it's dishonest to give 5 stars to lesser quality pics.