So I just realized that Steam has updated it's client so you can boot directly into big picture mode. Now if Windows 8 natively allows you to control your PC with the 360 controller from the Modern UI, then I can see myself putting a steam shortcut and then using my PC like a console. No mouse and keyboard required from booting my PC to playing a game.
I remember this wasn't supported when Windows 8 launched. Has it been added in yet? This could be the only reason I would ever upgrade from Windows 7 to Windows 8.