The idea of Xbox SmartGlass is really a combination of things. First, it is meant to be a cross-platform input and display device from any mobile device for your gaming and TV web surfing (thanks to Internet Explorer also coming to Xbox). What is unique is that the software will extend beyond Windows 8 devices, and cater to iOS devices like iPhone and iPad, as well as Android phones and tablets. It was also demonstrated to be able to pick up a movie where you left off on a tablet or phone and finish playing from the Xbox in your living room. This kind of playback sync requires you to be playing said media content purchased through the Xbox Marketplace, which is unfortunately only available through Windows devices and Xbox 360 consoles. More information about the media can be then displayed on the phone or tablet about participating content, such as interactive maps and IMDB-like info. Pretty slick.
In the former implementation, since this project was started about a year ago, it is pretty clear that the announcement of WiiU at last E3 was at least part of the inspiration. With Nintendo leading the charge in the motion gaming arena, all eyes from Sony and Microsoft were on them. Giving other console makers a year in advance heads-up now seems to be a rather confusing decision on Nintendo's part. Sony clearly also has an answer with their recently released PS Vita. Again, the difference with the other two and Microsoft is that the SmartGlass approach lets you use a device that you likely already own. It is also likely that Nintendo is helping the other two devices insure that multi-platform developers already have multiscreen implementations in the works for their games. The main negative that I can see with the SmartGlass version is that the other two already have dedicated controllers built into the touchscreen devices, while MS's version seems to require you to pick up and put down the alternate display/input device. The only solution I can see would be a split controller with varying sized docks or a wrist strap for your phone. Since the tech is still young, the answer won't likely arrive until the next generation Xbox hardware is unveiled, which is likely next year.