When I first joined Deviantart I thought to myself "this'll be great! A community of artists where I can get critiques from people who know what they're doing!" Critique is one of the most valuable things to a serious artist, a good critique can lead to new ideas, improvement in your technique, or style, and praise feels deserved when it is phrased like so: "I really like your use of color and line quality, but I think the over all composition could be better. What if you moved the subject from the center of the image and offset it a little toward the upper right?" Such a critique offers praise, tells you what you're doing right, and suggests a change to improve future work, and even a way to change it. This is a useful and helpful critique.
The comments I see on work on this site are 9 times out of ten not critique, they say things like "OMG so kawai! <33333 yuuuu!!!" yeah... what's an artist supposed to do with that? the first is great for feeling good for the moment "hey someone thinks I'm awesome!" great, and you'll keep producing the same thing over and over and over with no improvement. There is no value in that, you'll be stuck in a rut, and continue to stare at the great artists on Deviantart in wonder and despair because you have no idea how to do such work yourself. Why? because you get no helpful critiques or hints or tips on your own work.
An even worse type of "criticism is such comments as "Your art sucks and you're a terrible person" Not only is such a comment not constructive but it's an attack on the artist themselves. That is never OK in a critique. Critiques should be objective, constructive, and overall lead to an improvement in the artist's work.
The potential for good solid critique is incredible on this site, but it is never realized for whatever reason. Maybe you're afraid you'll discourage the artist from ever working again, maybe you're afraid their friends will all see your critique and hunt you down in the night, maybe you're afraid the artist is the boogeyman and he'll get you from under your bed. Whatever the reason for not properly critiquing on this site it is senseless. There are so many advantages granted by the internet; anonymity, the variety of styles, mediums, and experience in the artists in the community, I am truly disappointed that this site is not what it could be.
I know there are not many people who read my journal, probably due at least in part to my epic fail at updating with any sort of reliability, but anyone who does I would encourage you to really look at the work you are viewing and (if it is asked for) give an honest, helpful, constructive critique. We all have more to learn in our work, we'll learn new things until the day we die.