SmartGlass is totally revamped on Xbox One versus Xbox 360. Because Xbox One was "designed from the group up with SmartGlass in mind," it will be about 3.5 times faster than it was on Xbox 360. The reason for this is the console's ability to directly communicate with your connected device, versus having to go through an intermediary server.
The new SmartGlass experience requires a new app, but it will be available across Windows 8, Windows Phone 8, iOS, and Android. In addition, Xbox One will be able to support up to 16 SmartGlass devices simultaneously.
By using SmartGlass, you'll be able to access new features in a game. For example, Dead Rising 3 has exclusive missions that can only be accessed by using a connected device. However, something that should apply to every title is "Game Help." Microsoft's Ron Pessner explained the feature in a post on Xbox Wire: "During gameplay, SmartGlass follows your progress and knows exactly where you're at and where you're having trouble, so that when you hit the Help button you'll be given the tips you need."
Pessner believes that games can eventually use SmartGlass as a controller, due to the decreased latency of the application. However, "it has to be the right fit for the right game to use SmartGlass as a dedicated controller." It'll be interesting to see if SmartGlass will be able to one day usurp Nintendo's asymmetric play with Wii U.