resizable Live Tiles, Bing Wallpaper, new boot screens and new wallpaper. There is some speculation that Nokia will add new features to the camera on the device via Nokia Collection not too long after Windows Phone 7.8 goes live.
If you do have an AT&T model of the Nokia Lumia 900, you might lose your LTE connectivity if you try to flash your phone with the update. Best to wait for the official version to come, and you know that it will arrive eventually. Meanwhile, things are moving forward and Windows Phone 7.8 is expected to be sent out early in 2013.