So is the office that Booker keeps getting sent back to supposed to imply that he has died (or at least failed) and the game is continued in the next loop? It happens when you die during gameplay and Elizabeth is not around, you reenter the game through the door and it happens several times during the events of the game, usually after something somewhat traumatic. The twins and old Elizabeth suggest that Booker has failed many many times before, so I guess the question is whether we as the player are spanning multiple parallel multiverses (or whatever) over the course of the game or are we playing as a consistant Booker who is "the one" who is finally able to break the cycle?
It was a fantastic game, but I'm disappointed by the apparent lack of new game plus (1999 mode is just a difficulty adjustment right?). Seems like they could've dont something really cool with a new game + that could somehow incorporate the idea of this infinite loop, work it into the fiction. If nothing else I'd like to carry over all the vigors/gear so I can continue to unlock new stuff.