First note: Visiting Rapture was pretty cool, and of course significant. "A city underwater? Ridiculous." It's also pretty clear that Songbird was inspired by the Big Daddy models from another reality, or at least, might be. Songbird itself might need some explaining through DLC, but I'll have to do another playthrough to make sure I didn't miss anything. Was a Booker DeWitt controlling Songbird?
Second note: This whole alternate reality stuff was really well done, I think. Booker being reborn into Comstock, into some sort of loop, was very interesting.