I'm quite against a separate forum for new members.
A new forum (or even just a thread for developing ideas) on the other hand could be worthwhile.
Yes, the judging can seem quite harsh, but it's generally fair, and provided the mocker actually listens to the advice initially given, they'll get constructive feedback. The absolute worst things you can do with a new mock is: a) demand people only give you constructive criticism, or shut up; b) continually post new ideas seemingly unrelated to your previous work; or c) give up after 24 hours and start a new thread with something completely different. Mocks which people put time and effort into are scored highly, things that have taken five minutes to scratch out in powerpoint will always be voted poorly - a new forum for new members will not change that one bit, and could even make the situation worse if they get highly voted (by who?) mocks which then get moved to the Gallery.
So, yeah, er, a "The Scratchboard" thread in the Gallery, for anyone to post ideas and get feedback may work and be helpful, but a whole new Gallery ("The Crèche"?) for newbies only is a stupid idea.
It's spelt AERIAL!