Just my 2 cents worth...
I like to keep in mind that "success" in a photograph (in my opinion) is a matter of whether the photographer achieved the look he/she was going for or not. If your goal was to create an underexposed, blurry image and you used whatever technique to accomplish that look - then you have a successful photo - (but there may not be many people who LIKE it) If, on the other hand, your desire was to create a classic "Ansel Adams" landscape and your result was a blurry, underexposed image, then you need some CONSTRUCTIVE critiscism - which to me would sound something like: "I think you may want to try a smaller arperture and put your camera on a tripod - then try metering off of some medium tone or a grey card to get your exposure. Compositionally, I am having a hard time understanding exactly what the subject is - if there is a way to define it in you composition (different angles, tighter composition, etc.) it may make a better image - but that's my opinion" - to me, that give the person better information than "Boy, this really stinks - why don't you sell you camera and purchase an Etch-O-Sketch!"

If you have given someone reasonable advise as to how to improve an image, and they are offended by it, then they either are too full of themselves to admit they're work could be improved, or too thin skinned to be posting their images to begin with.

There are several "styles" of photography that aren't "my cup of tea" - but I can appreciate a well executed photograph....even if I don't really "like" it - if that makes any sense.