I notice this thread from Newsletter, and it got my attention since I’ve had exactly the same question about my last submission:
https://www.cgsociety.org/cgsarchive/newgallerycrits/g45/416045/416045_1318668848_large.jpg ,
which was rejected, despite the fact some earlier works of mine which I think are worse were instead accepted. Previously, I was also surprised to hear that even this one of Narcius:
http://narcius.cgsociety.org/gallery/1004361/
didn’t make it, and it is also much better than many other in the Gallery, so I wondered if the judging criteria had changed and how.
I also received exactly the same email message as Pierluigi did, and, to those who said that constructive critique must be accepted (which I agree with!), I must anyway say that this kind of standard messages are clearly not helpful to understand why the image didn’t come in, because they don’t really account for the specific flaws the mods found in the submitted picture. The explanation, in this thread, about how mods work and why is much more informative than the standard email sent to people.
At DeviantArt there are also groups who accept or deny pictures on a quality basis. Normally there are 3 judges and majority decides. They’re also doing this job on a volunteer base. The difference is that you can send a comment and kindly ask for feedback when a image is denied, which is very helpful. I can understand that maybe it’s difficult to have something similar here, but in this can be better to just give a link to “how the mods work” as other suggest, instead to send a mail message which is not that helpful.