In offering my opinion about a photograph (if asked), I try to start on a positive note, finding something good about it. I move from there to asking questions about the thought process behind the photo, if for some reason I see something that makes me shake my head and wonder, "what were they thinking??". I typically do NOT express my opinion on photos that I can't find something positive to say - perhaps it's a blurry, camera phone "myspace" (arm-length self portrait) photo that someone asks me about. In reading critique about my work, I like to read what someone thought about the work, and an explanation of what they would have done differently.
The only time I really take offense is if someone simply says "It sucks", and can't offer any reason why, or what they'd do differently.