The most prominent of the features is the brand-new 3D engine that allows Marathon: Durandal to run at 60 frames per second, compared to the 30 frames of the original. As you can clearly see in the image above, the leap in quality is tremendous. The jump from using a keyboard and mouse to the Xbox 360 controller has already been bemoaned by some, but the developers at Freeverse have been aided by Bungie in their pursuit of FPS perfection. Online play will have a huge role in Durandal with support for up to 8 players in both cooperative and competitive modes.
The new Survival mode is laid out and described by Freeverse's Bruce Morrison as, "a giant truck that pulls up to your house one day, where a platoon of alien commandos jump out, kidnap you, and wail on you until you're dead. And then the truck explodes, just to make sure the job gets done." Expect to be faced with wave after wave of enemies by the dozen in a desperate struggle to conquer the leader boards.
One feature from the original Marathon 2 getting the axe is saved films. A prominent piece of the Halo 3 puzzle, saved films simply wouldn't work out in the XBLA release of Durandal due to both the new network code and lack of online storage afforded to a game of its kind.
As to why Marathon 2 and not the original, Bruce Morrison from Freeverse gives two answers, though the non-technical of which is easier to digest. Should they have been faced with the worst case scenario of only being able to release one title from the series, Durandal simply made the most sense due to its comprehensive campaign, richer story and technical superiority over the original.