The original Xbox button directed you to the guide, now it's the dashboard. It minimizes everything while it's still running in the background, and has a lot more options now. You have to ability to play/pause your music, access your games and apps, and your home menu. Hopefully this will start up pretty quickly and have no performance issues.
Via Videogamer
Xbox One is dropping the Xbox 360's 'Guide' interface, Microsoft has confirmed.
Instead, pressing the Xbox button on the Xbox One controller will now take players directly to the dashboard, shrinking down whatever content they're currently running to the background.
Via Videogamer