LiaThistle
Actually, since it only says "Restored," I'm under the impression that it's more like Avacyn herself has been restored. As though she were fractured into the demon and her angel self, and now she's back in one piece.
Which would be a potential explanation for the scary looking armor that angel has in the image.
It'll be nice to finally have them end the constant speculation of "is Avacyn a Planeswalker?" and "is Avacyn Griselbrand?" But it would have been nice and simple if she were just gone. Serra and Asha both died, and things didn't go crazy. I remember it wasn't quite clear if Asha was still alive before the Conflux, and there were some theories that she was taken to another plane by Malfegor (hmm, an angel missing after fighting with a notorious demon... does this sound familiar?). But then it was revealed that she was dead, and that was actually part of what got Bant into shape to fight in the war. And the jury's still out on Serra with the whole Karona mess, but a multiplanar religion doesn't need their goddess to be alive. Like she said herself, "Believe in the ideal, not the idol." Point is, Avacyn's death could have been used as a strong plot point.
Liberi Glacialis
I think Double Faced cards will stay. Flashback has a shot, but I think the whole Morbid theme is getting dropped/swapped out.
Large sets never share mechanics, at least not back to back like this. They do it to make sure each plays like its own distinct environment without crazy amounts of overlap. I think they did a great job of making Zendikar feel different from Rise of the Eldrazi while making it clear the setting was the same. Hopefully they do the same with Avacyn Restored. Hopefully this set actually makes the plane of Innistrad interesting.