"Gimmefeedback" is a group dedicated to a system of trading feedback and suggestion on ones art. The group is hosted in the group system on Deviantart, I was recently promoted to an administrator ("co founder") with them. It has been around for a while, I highly recommend it if you are looking for a way to improve your artwork. At this stage, there is mostly beginners helping one another out, but with more attention, the quality of feedback will go up as more experienced artists participate.
A lot of groups on deviantart are purely for showing off work, but our group is purely for improving work, and thus accepts all quality of work as long as the artist wish to evolve. We run competitions, feedback events, and teaching sessions.