The most notable addition is support for Xbox SmartGlass "second screen" functionality. These "interactive experiences", which are associated with select game and entertainment content, provide contextual information and features (secondary mini-games, maps, associated content). There are relatively few of these currently available, but more are expected to be added in time.
One example, which is available now, is for the Xbox 360 game Dance Central 3. The SmartGlass experience, Dance Central 3 DJ, can be used, in multiplayer mode, to queue up the next song and switch difficulty settings. In single player mode there's a calorie tracker that keeps a tab on how much energy you're using over a set period of time.
The SmartGlass app can also be used as an interface / input mechanism for web browsing on the Xbox 360. The phone's touch screen effectively becomes a laptop-like track pad, which can be used to move around, and zoom in and out on, a web-page, and navigate to specific links. On screen buttons act as the equivalent of the B, X, and Y buttons on a standard Xbox 360 controller, and the phone's keyboard can be used for text entry (e.g. typing in a web address). In takes a bit of getting use to, but it does make for a more intuitive browsing experience than when using a standard Xbox 360 controller.
Xbox SmartGlass lets your Windows Phone work with your Xbox 360 console to bring rich, interactive experiences and unique content about what you're watching or playing, right to the device that's already in your hand. Interact with your favorite TV shows, movies, music, sports, and games, and bring remote control to a whole new level.
Xbox SmartGlass can be downloaded from the Windows Phone Marketplace for free.
On a related note, there's also a SmartGlass app available for Android device, available via Google Play, reflecting Microsoft's stated intentions to make it services and key user experiences available on as many platforms as possible.
Here's a video showing SmartGlass in action on both a tablet and a Samsung ATIW Windows Phone device: