It was pretty obvious that Irrational would do something with Rapture again, what with the ending of BioShock Infinite, and the negative criticism from everyone who failed to understand Columbia. Clearly, people want more Rapture.
So Burial at Sea takes place just before BioShock, in a Rapture that isn't entirely dominated by splicers and creepiness. It's interesting to see expanded parts of the city, when they were "normal", and the architecture is beautiful. However, as some players have said about Columbia, it kind of just seems like a museum. There are plenty of people about, talking to each other and having interesting conversations, but you still feel like you aren't interacting with anyone unless the game specifically wants you to.
Granted in BioShock you literally never interacted with anyone except Dr. Tenenbaum, not in a meaningful way, it still feels like you should be able to. Due to the newness of the situation, this Rapture looks like the Rapture you love, but doesn't feel like it. You are used to a rubble strewn, burning, dark and crazy city - not this bright, shiny paradise. Even Fontaine's sunken department store, which is much closer to the original Rapture environment, still seems kind of tidy. Not to mention the city suddenly has these "pneumo lines" that you use to take advantage of the sky-hook mechanic.
I wouldn't say Burial at Sea is bad, because it's certainly fun, but it's hard to judge until the last part of the DLC comes out. One thing is for sure, the ending will make you have to do some thinking again to understand how it fits in with the main game, or if it even does. Spoiler alert: The end is awesome, but in a way it does seem to supplant and cheapen the original ending of Infinite which was a self-contained deal - assuming that the theories about the DLC are anywhere near correct.