Matt Tabak has posted an article detailing the new and returning mechanics in Shadows over Innistrad. He also details some rules changes to double faced cards and transforming.
Check out some of the changes below:
Under the new rules, the converted mana cost of the back face of a DFC is based on the mana cost of the front face
No more Ratchet Bombs to wipe a field of DFCs!
if a DFC has an activated or triggered ability that transforms it, that permanent transforms only if it hasn’t since that ability was put on the stack
This is especially relevant in light of the new Avacyn that was spoiled, since there was speculation that she could flip over and over if multiple creatures died in the same turn and deal an enormous amount of damage to your opponent. This means she only transforms once.
Read the rest of the changes on Wizards of the Coast’s site here.