I like the idea of a critics/WIP forum. One advantage is that you can have an ongoing "conversation" with people and have the previous comments handy while replying. Unless they changed recently, the Galleries only allow the posting artist to keep multiple entries per image. Anyone else will have thier earlier comments deleted. It's bit of a pain when having a public conversation, as opposed to a private one with IM, and you don't have previous comments.
True there is IM for such discussions, but sometimes a 3rd party can chime in with some helpful ideas.
Just a thought