By critique, I don't mean snarky comments about not quitting their day jobs, but a thorough written statement that starts out with what they're doing right before pointing out what they can improve upon. The word wrong should never be used in a critique. Period.
In his brilliant book This Won't Take A Minute, Honey, Steve Almond talks about his own revelation:
My critiques sometimes ran longer than the stories in question. A number of my comrades found this behavior presumptuous. I, on the other hand, assumed I was being wildly generous. We were both wrong. As it happens, I was explaining to myself how to stop sucking as a writer.
I have found a couple of pretty good online critique sites and I always give at least five for every one I get, because I've found it helps me exponentially to do so.
If you haven't tried giving a critique, don't be shy. Take a deep breath (at the very least your chest will look bigger) and jump on in. Even if you are a non-published writer your opinions are valuable. Most writers know that, and they appreciate any and all feedback they can get on their work. I know I do.