For one, there's two efficient axes of navigation along the Metro start menu, you can go left-right or up-down with your keyboard, which is SO much more amazing than the "hold down down key for 6 seconds. Awp go back up five. Now hit right four times. Enter" to launch an app that is actually "pinned" to your start menu. I can orient my Metro tiles how I want, and I know I can get to it within 5-6 arrow-key presses max. This is very nice and for the first time in 6 years I've actually used the Windows key to open the start menu for something other than shutting down (have been for the last 18 months of using Windows 8.x).
Going full screen really doesn't bother me one bit. I'm launching a new app, I'm losing context anyway. I find this similar to Apple's launchpad or whatever that OSX thing is that I never use because Spotlight and Terminal is all I need from that OS.
Vaguely related to improving productivity, Win8 has significantly better multi-monitor support out of the box, with all the flexibility I'd want, and with it better window management, than either of OSX or Win7.
Just my $.02