This is what I wrote in response to a similar post in another forum, Ruben:
The art of the Critique
Most of you have no doubt noticed the section under each piece of artwork in your individual galleries that is set aside for people to post comments. No doubt most of the comments you have received are things like: "Gee, this is good!" or "Very nice work!" and so on. These are comments, not critiques. While these are nice to look at and make you feel all warm and fuzzy, when it comes to helping you develop your technique and helping you to grow as a photographer, they are almost worthless. What is worse, the general effect of most of them can be to encourage complacency. There is a difference between a comment and a critique. Critiques are far more valuable. I'd trade all of my comments for one good critique.
A comment is a general statement of approval or disapproval. "Gee this is so good it makes me want to have your baby! You are a photography God!" is a comment (a somewhat disturbing comment, but a comment nevertheless). It does not mention what the person found admirable about the photo. Similarly, "This sucks. In a perfect society, real photographers would gather together in order to beat you to death." is a comment. It does not say what in particular the viewer found objectionable. Comments leave the artist wondering why the photo is good or bad and, most importantly, they don't pinpoint areas where he needs to develop his or her level of skill. I'd like to stress this, because this last is the most important aspect of a critique. If you don't find something that needs improvement, point it out, and suggest a way to improve it, you have left a worthless comment, not a valuable critique.
Now compare the comment to a critique: "I like the general effect, and, in particular, I like the detail in the foliage. However, I noticed that the bright red of the lady's fingernails draws the eye strongly, and distracts the viewer's attention from her face, which I feel should be the focal point of the composition. It seems to me to be a little overexposed, by maybe one f-stop. When photographing a model against a black background, you need to meter on her skin or use a gray card. A meter assumes that all scenes are supposed to be 18% gray and it looks at the whole scene and gives you a reading that will give you the "correct" exposure to achieve this. With this much black, the reading will be incorrect. However, most Caucasian skin is pretty close to 18% gray and if you approach the model so that her skin fills the viewfinder, you will get a good meter reading. If your model is Black, you can meter on a gray card in similar lighting. The white border is not really working either and I think it makes the photo look flat. This has potential, but isn't quite there and I'd reshoot it." A critique requires a little analysis, and it points out a photo's problem areas. This gives the artist an idea of what areas of composition he or she needs to concentrate on and aids him or her enormously in developing artistic and technical skills.
Now every artist has (or should have) a different style (and maybe a whole different interpretation of art), and so the photographer being critiqued may not agree with you. Nevertheless, the critique is still valuable - even if our hypothetical photographer rejects your opinion utterly - because it allows him or her to see how others perceive the artwork in question. If people are missing the point, the photographer can work on a way to emphasize it a bit more in the next shoot.
All this said, the purpose of a critique is to encourage the development of an artist's skill, not to rip his or her work to shreds. You start with a compliment and point out just a few problem areas, even if you think the work is a disaster and riddled with them. If you mention too much that is wrong, the general effect will be to discourage the artist from creating more work instead of to encourage him or her to create better work. This is what is meant by constructive criticism. If you are not very good about compliments (or can't find anything to compliment), then at the very least start off with something like "No offense, but..." so the artist will know that he or she is not being attacked.
ALL photographers who are at all serious, and who are worthy of being called photographers, will sincerely appreciate critiques. It is worth the effort to supply one and you will often find the favor returned if you go to the trouble. What is more, analysis of other's efforts will be of benefit to you in analyzing your own work.