According to the
latest Catacysm patch notes, "Frost Armor is now trained at level
54. Up from 7".
So levelling mages who aren't frost now have one of their main means of defending themselves from incoming mobs removed. This won't affect raiding or instances, just a mage's ability to level.
Now betas are the right place to be experimenting with different game balance options, but this one is so bizarre that it's unfathomable. How did the design review meeting go?