Picked up this book a few days ago at the local in Glasgow. It's... it's an interesting step GW is taking. First of all, 40k is definitely getting the AoS treatment in terms of rules direction but also in a fluff direction.
We're getting less of an Order/Disorder smackdown so much as Warp vs. Reality split. Since all races will be playing a part I'm interested how the Tau will do anything meaningful... I really hope they don't answer the question regarding the Old Ones, or Eldar, having anything to do with their creation... It would be a 'why calmsword plays' killing decision. That and the Tyranids, I just don't see how they'll be involved.
My assumptions would be that the war goes Galaxy wide. Imperial and Abbaddon agents seem to be everywhere, trying to gain artifacts or silence them which will have an effect on the greater war. The Orks, for example, start the book by engaging Cawl as he tries to recover information about the Pylons on a planet light years away from Cadia. We might get the same treatment with CSM going to Arthas Moloch for some kind of Daemonic artifact or something.
I had initially written a chapter by chapter breakdown of the book- but ATT logged me out and I lost the entry (seriously, how does that work?). Instead I'll make some general comments and if anyone wants to ask any fluff specifics I'll post them: