Is it impossible to get valuable critique on such a forum?

I'll say most is valuable. It also depends on the person it's coming from. I know your work, and I understand your critique even if it can be harsh at times. I don't mind it because it helps me see the shortcomings of something I've produced that I might not have seen. Then there is the ones that are just meant to be hurtful which can be discouraging to some people, especially those just starting out. I've even had some fool dive into my gallery and pick out some shots I had put there to show someone a particular gear problem I'd been having and then post on here that my photography wasn't worthy of owning a D7000. For me I didn't care, I'd been around photography and forums long enough that fools like that aren't worth bothering with.

Now though if I want honest opinions I'm more likely to put something up on a gallery and ask friends who will give me an honest opinion and not pull any punches. Mako is much too easy, but Lacie, oh man...she literally chews up bad pics.

I like when people ask for CC on their pics, as long as they want a true opinion...but heaven forbid if you ever say anything bad, especially if it's a shot with a kid in it. Your horns sprout, your pointy tail emerges and people go running for the pitchforks.