I'm not sure if it's a matter of preferring a controller. I play games at a desk, but also on a PC connected to a TV in my living room. At the desk I use keyboard/mouse, but in the living room I use an Xbox 360 controller so I can kick back on the couch and play. This obviously works better with certain types of games than others. Some games are no fun with a controller.
Me either...I wasn't getting that implication, though. Valve designed a controller that they claim is supposed to make it easier to play games that you'd typically need a mouse for, so maybe that's what people are on about. It sounds interesting, but I'm skeptical of a controller that relies on a trackpad.
On top of higher resolutions and framerates a better GPU should allow you to enable more eye candy (nicer lighting/shadows, nicer textures, ambient occlusion, AA/AF, tessellation, etc.) if the game engine allows for it.
IMO the streaming of games from another PC seems like the most interesting feature. You could potentially get a really low-end (and cheap and/or older) machine running the free SteamOS and stream games from a more powerful Windows machine in another room (that runs Windows games). In theory it'd be like an extension of your gaming rig into your living room without buying another full-on gaming rig. It could go nowhere, but it could also be pretty convenient if it works well.
Interesting. The vast majority of previews that I've read were based on beta hardware and the reviewers were cautiously optimistic about it. IMO that's actually pretty encouraging for something that people have little experience with and that's such a departure from conventional input devices. But hey, he used it for a whole 30 minutes, so he's probably the final word on the subject.