Crysis 3 developer Crytek had to "rip the engine to pieces" to get it to work on the aging console hardware.
In a Q&A on Monday, Mike Read, producer of Crysis 3 talked at length about some of the development challenges facing the Crytek team. The frontrunner here was, as you may have guessed, the nearly eight-year-old console hardware which presents a "massive barrier" for the Crytek team. Their first foray into utilizing CryEngine 3 on consoles was Crysis 2, and the learning experience should allow them to push the boundaries of the current generation with Crysis 3.
When asked about the various changes noticeable in the Crysis 3 trailers, Read mentioned that "Crysis 3 has allowed us to make a lot of tweaks to the engine itself." After having to "rip the engine to pieces" to get it running on console hardware, Crytek has apparently been reassembling it piece by piece, making the requisite tweaks along the way.
Read also said that, while players may expect Crytek to build Crysis 3 from scratch, that the end result of this approach just wouldn't really "be Crysis" so they're opting to find some middle ground between the "openness" of Crysis and Crysis 2. Read expects that players will still experience "wow" moments, given the vast cityscape of the New York City setting of Crysis 3, adding that Alien weapons are coming back, as well as some updates to the perk system which should make the multiplayer easier for new players to pick up.
There is also the updated AI system, which was noted as being a "big challenge" in development, and took a huge amount of work. This dynamic AI is touted as something to watch, however, and when Crysis 3 comes out, you should be able to experience the dynamism for yourself by simply repeating a section of the game, and seeing how differently the computer-controlled characters can behave given a slightly-altered situation.
On the sometimes touchy subject of DLC, it is apparently being planned for continuing support of Crysis 3 after launch, but the details of any DLC, day one or otherwise, are currently unavailable.