Criticism always serves a purpose and it goes beyond tough/easy/positive/negative, the purpose is to revise and grow. After writing (not professionally) for several years I know that my writing has improved but it goes through leaps and bounds of improvement when I get something that is more than "Whoa, that was great, just fix the grammar and you're good to go." We all love those types of critiques, we do because they make us smile and hold to the belief that we can do this crazy thing called writing. But it's not always the best for our writing.
There's a lot to critiquing as other's have said and doing it well. It is a learned process, just like writing. However, if you are going to post things on the world wide web, then I have to say be prepared for people to comment who think they are writers and give good critiques. May be they are just being jerks, and want to tear you down, but perhaps they haven't been taught what they should say. I know it hurts, for a moment. Just remember, they took the time to read it, AND to comment. That says something. That's what I told myself with every negative review I received in fanfiction (I know not my original stuff, but I put work into those projects). I pushed someone so far, that they had to tell me personally how much it angered them. Sure I wanted to know what I could have done better but hey they read and told me their opinion, lucky me. I knew great writers who never even got that.
It is a little odd to think of it like that, but they read it, some agents and editors won't ever read what you wrote. They'll just reject you before they even see page one.
I will also say that editing/critiquing online is difficult. I prefer to have the document sent to me so that I can mark it up and make comments directly on the page. So I have co-opted several friends/acquaintances to read/tear-apart-like-a-rabid-dog my work and it's been the best and worst thing in my life. I love getting them, I am sometimes fearful to read them. But afterwards I write for like ten hours and the story comes out beautiful.
After two writing classes, one taught by and editor, I've come to love the editing process more than the writing process. Because most authors will tell you, books are made in the editing process, not in draft one. Editing gives you critiques, I got one from my friend that said I made her biology heart SOB, and not in a good way. That I was failing on a huge part of my mythology/science, and it was so bad she ranted for a few comments and then just put 'see me in person.' That scared me. I read every line she wrote me, and it hurt. A lot of a it hurt because I loved what I wrote. But I took what she said, and we had an entire discussion about genetics. In the end, I grew more and the story itself became better, my writing was tighter and it was all because she was brutal. And I asked her to be. I ask all my critique people to be. For me, it's the only way I see to grow significantly as a writer.
It may be hard, but my belief as an artist is you have to be able to take criticism. Period. End of Sentence. If you don't have skin as thick as a dragon's hide, you might as well throw in the towel right now. There are rejection letters from agents, editors, and a million other places all ready to tell you how you fail. Trust yourself, trust your characters, and be prepared with a sharp sword to defend yourself against the onslaught of the publishing world.
