There are a ton of young people on Deviant, and the simple truth is that many of them, although they may possess well developed opinions regarding critiques, DO NOT understand how they are used or applied in the real world.
School is not the real world. Neither is home, and the web is DEFINITELY not the real world.
What is the purpose of a critique?
Well, one purpose it is not required fulfill is to make you feel good about your work. If the sole result of a delivered critique is that is makes you feel good about your work, it is not a critique, it is an ACCOLADE.
A critique serves two principle functions in the real world.
One; it serves to educate. It points out areas in your work that need attention and possible improvement, and, at the very least, may deserve further practice or repetition on your part to perfect.
Two: it serves to describe the differences between how you see your work, and how another sees it. This can be also very educational to anyone who truly attempts to derive the most from any opportunity to learn more about their craft.
What form should a critique take?
That is entirely dependent upon the circumstances or venue in which the critique is delivered. However, any true critique is inherently negative. If it were not, it would not be instructional. There is no purpose in informing you, the artist, about those aspects of your work you got right.
Most students are coming to grips with their own view of their art. Couple this with the fact that even many successful artists remain insecure about the quality of their work, and it becomes easy to see why most artists are touchy about critiques.
This is a bad thing, and cannot be justified by anyone claiming to be professional in their work. Whatever else you may justify regarding how you view your art, you do not have the right to ignore or limit input on it from qualified sources simply because you have an innate need to always "feel good" about yourself or your work.
Just about nothing you can do as an artist is LESS professional.
Let me offer a small warning to all you beginning artists out there:
The closer you get to actually making a living with your work, the more realistic and thick skinned you better become about getting critiques. I am an art director. I have been one for more than 15 years. Over the years, I have worked with dozens of young professional artists, designers, and illustrators. More often than not, those young talents have left my service to go on to make more money and enjoy more prestige within their field of work than those that have not. So, I consider myself to be both a successful art director, and a pretty good critic.
Working critiques are to the point. They are almost always delivered by someone who knows more than you do about the kind of work you are trying to do or learn. A working critique is blunt and precise. The point is to make surgical corrections. In a working environment, time is the most important factor in any successful design job. This is true when delivering critiques as well.
Depending on the job, "not good enough... do it again." is an acceptable critique.
Generally speaking, working critiques are designed to get fast, predictable, measurable results. As an art director, I do not have time to take an artist's feelings into consideration. I do not mean I am harsh simply for the sake of expediency, I mean that the primary goal is to make the "job" work out right, not re-enforce the ego of the artist.
It is also important for young artists to remember that the person delivering a critique to you, once you leave school, is almost certainly a lot more experienced than you are, and is also paid to oversee what is produced in the art department. Their opinion is almost certainly guaranteed to be valued more by ownership or upper management than yours is. It is in your best interests, both as an artist and as an employee, to listen to any critique of your work offered by such a person.
Bottom line: get really thick skin. Learn to depersonalize your work. Treat it as a commodity, even if you are an amateur or not a commercially oriented artist, and you will get a lot more out of critiques than you are now. Take the hits, review your work after getting them, and try to limit your defense of a piece.
While it is not universally true that defending a piece means you cannot accept a critique, it usually does mean that you are too busy defending to really learn what is there to learn from the critique in the first place.