I'm glad that people are delighted that the next book is on the way overall. GRRM and Parris themselves were worried about the reaction on the board. GRRM even mentioned yesterday that he heard that people weren't happy here. But flicking through a couple of threads I think that, ignoring a few, people are just glad that the new book is finally on the way. :D
GRRM will tell you himself its not an ideal situation. But its definitely the best way to go IMHO. (The ideal situation is that he spends years working on the whole series until it is completed. And not be pressured by publisher and fans' demands. Because of course he feels a certain obligation to his fans. :)
Just to cover a few things that I saw mentioned...
The focus in FEAST will be on Westeros, King's Landing, the riverlands, Dorne, and the Iron Islands.
GRRM said he is unsure why he has kept the Mystery POV hidden all these years but he isn't going to reveal it now. ;) But i'm thinking we don't have a POV directly set in the Riverlands so the quote above may be a hint. Of course, Sansa, Jaime and [Note: Redacted spoiler POV] can give us info on those areas. But we can assume that it is a southern POV anyhow (It isn't Theon for example but definitely could be Brienne or Sandor).
AFAICT the only Iron Islands chapters in aFfC will be the Arms of the Kraken story. Asha will feature in at least one chapter in aDwD (which is already written).
Arya will have 3 or 4 chapters (I can't remember exactly) in aFfC. But she will be one of the characters that will also feature in aDwD. We don't know how many chapters because he hasn't written them.
He says he has currently 600 pages written for aDwD. He is expecting a 1100 page book. (All in manuscript pages.) Which is just over the size of aGoT. He probably is underestimating again. But that gives you an idea. Tyrion's story arc required 4 chapters but he thinks with another 3 chapters he can have a far more satisfying story. In other words, he is just continuing the existing story.
Other POVs characters may also feature in both books. I get the impression he doesn't want to give away too much info about aDwD or he will end up feeling straitjacketed again. That he wants to avoid feeling he has to do something because he has already commented on it. Thus he is mainly commenting on what is already written.
This is actually one of the potential benefits of the books being done this way, but because it wasn't planned this way from the beginning, I'm not sure how much he can take advantage of it
GRRM himself stated this is an advantage of this book. That we will get some hints of what is going on in the North and East but we wouldn't know what is true and what not. We may learn that Stannis is killed for example but that could easily be a red herring. :p Look how Varys often manipulated the truth.
I asked GRRM about the title no longer referring to the Nights Watch and as people explained already, the title always had multiple meanings. So it merely loses one level of significance. And when I think about it, the Crows on the Nights Watch are unlikely to have much of a feast. Except of death. :p
GRRM did mention late September/October as a publishing date but nothing has been finalised. He has to meet his publishers. I think he is doing so this week in NYC. It sounds like they are trying to publish asap. His editor has read the new version of aFfC and has not come across anything obviously wrong with the new format but is currently rereading to make sure nothing is missed. GRRM did admit that a few lines may require tweaking to ensure he doesn't give away too much.
He has suggested there will be 3 or so extra Tyrion chapters, a couple of Bran chapters (he originally wasnt going to appear in the old version of aFfC) and a few more Arya chapters (to go alongside those that will appear in the new version of aFfC). They are not going to add up to 500 pages. He could be expanding a few other POVs that we know will be appearing in aDwD. Or perhaps others from aFfC will be appearing. I wouldnҒt be surprised if GRRM isnt sure himself yet. He probably has a few ideas but isnҒt ready to commit publicly.