One of the biggest new features is the addition of downloadable Xbox games, this will cost 1200 points per game ($15). Initially the service will feature games like Halo, Fahrenheit, Crimson Skies: High Road to Revenge, Crash Bandicoot: The Wrath of Cortex and Burnout 3 but many more games will be added later on.
Microsoft is also working to add a video-on-demand feature (with 720p video) for European users but unfortunately this won't be included in the December 4 update.