The key difference is that the Steam Machine can be controlled 100% for the controller with a OS built to be controlled by the controller, and seen on a TV.
Windows doesn't do that. It fuckin sucks to see on a TV because the UI is so small, and the controller only works in-game.
I am most excited for the OS progress. I will be putting whatever on my living room pc as soon as it's released. I can't stand using a mouse and keyboard in the living room and having to scoot close to the TV