
HD graphics and the ability to see the original graphics too on the fly, coop and multiplayer online modes added, achievements, skulls to collect that unlock tweaks to the gameplay just like Halo Reach, 3D if you have the TV for it, hidden video terminals that tell you more about the story, Kinect support where you can scan things and get more information about them, revamped multiplayer maps from Halo 1 and 2, and a new Firefight map.
In listing these I forgot how cool the Kinect support was. I might have to get that darn thing now.