Personally I don't care for the card pool changing mid-regionals; I've already played in one and I was planning on taking the same deck (with minor modifications) to another, but some of the new cards (yes, Incinerate, among others) leave some gaping holes in my otherwise relatively solid defense. Its somewhat disappointing that, given the amount of time I've put into it and the love I have for this particular deck, I feel the deck is much weaker now and probably not regionals-worthy.
The other issue I have is that rules may be unclear on "brand new" cards. For example, if Sorrowful Man was in the new BtNS pack instead of the last AToC pack, incorrect rulings would almost certainly have been made at some point. I always thought one of the reasons for keeping the card pool fixed during Regionals was to ensure that players had a chance to familiarize themselves with rulings and card text. Same for TOs… I TO'ed a Regional and I have a pretty comprehensive memory of both the cards themselves and card interaction (mainly from spending too much time on the forums… how many times can the same questions about Red Wedding come up? ) and I wouldn't have been happy walking in the door to TO when I'm not familiar with a CP b/c it just came out last week.
But ultimately the surprise factor of the announcement is the biggest concern… if FFG doesn't consider the surprise notice a problem, can we expect to see a new FAQ and new erratas mid-Regionals as well? How about changes to the Restricted list?