Sure, the first BioShock's theme of infanticide is pretty mature. But BioShock Infinite takes things to a new level by tackling some real-world issues—not that harvesting genetically modified little girls for their DNA currency isn't realistic. But it isn't.
You'll still be shooting plenty of people in the face (and burning them alive, and laying traps, and siccing murders of ravenous birds on them, and so on). But on a deeper level than that, BioShock Infinite deals with religious fanaticism and American exceptionalism—the idea that America can do no wrong—not to mention good old-fashioned racism.