I just read though this thead, some very interesting comments and I agree with them. Critiquing really doesn't reguire a person to be an exceptional photographer, photo editors that work for magazines and newspapers aren't always the most skilled photographers and many aren't photographers at all, but they do know a good photograph, they also know how to destroy a good photograph if they don't understand all the componants that make up a good photograph, composition being the big one.
I've spent a lot of time looking at photographs and figuring out what I would have changed if I had shot it, but one factor comes into play in a lot of them, what limitations were put on the photographer while he was shooting the photo, example, you look at the angle a photo was shot at and say the angle is wrong, he should have moved 2 feet over, what if the photographer didn't have the 2 feet to move over? At that point we have to just take it at face value without knowing the surroundings involved in the creation of the photo. It's the best that it could be from where it was shot and do factors like that come into play.
As has been said already, there are a wide variety of people on this forum, everyone has an opinion, some of the opinions are directed at the person who posts the photo without regard to the photo, it wouldn't matter how great the photo is, the personality behind the critique will be negative regardless. This is where people have problems with negative critiques, they aren't even critiques, they are just insults. Personally I don't care if someone looks at one of my photos and says they hate it, but tell me why. The word "sucks" isn't a word used to critique.
Some people just can't put into words why they like or dislike a photo, I look at some photos and like them, it's that simple, it's a combination of factors, if someone wants a more detailed reason, I can pick it apart and say why I like it.
This is an example that follows the same line but different subject. I've worked as a car show judge for over 20 years, I get to go through show cars that some people have spent hundreds of thousands building and find flaws. I have to provide a detailed breakdown on what I think the car needs, or find the weak areas that can be improved on. So I get to tell someone that has 500k invested in a car that he should have buffed out his 40k paint job in a couple of spots, he may not see it, or chose not to see it because he feels the paint is perfect, but it's not. Does he really know that it's not perfect, yes he does, but he doesn't want to hear it, it's negative.
Same as photos, we look at our work, and think it's perfect, but it takes someone else to say why it's not.