So I've set up an HTPC box recently with Win 8 / Steam Big Picture Mode that works for gaming with a wireless 360 controller.
Random thoughts:
Steam Big Picture Mode is really good and works perfectly with a 360 controller. Although possibly partially as a result of the Steam desktop UI being kind of poor, Steam BPM is probably the app I have that does the single best job of transitioning to good TV UI.
I still don't see the point of Steam OS. Save $100 to not be able to run any desktop apps?
I don't get the point of media UIs. I tried XBMC, Plex and the Windows Store Video app (which now supports MKV). None of them are easier than just leaving open a File Explorer window open for every show I'm currently watching. When I want to watch the next episode of something, I alt+tab to the appropriate window, push the down arrow, hit enter. Process takes about 2 seconds from anywhere else.
I never use metro apps on Win8 other than NextGen Reader (which is awesome), but they're actually pretty great on a 55" screen from across the room. Metro Internet Explorer is my browser of choice on my TV.
The quality of Windows DPI scaling compared to Mac OS is really understated. I run at 125% on my TV, and it's mostly fine. There are some poorly maintained desktop apps that are blurry messes, but the same thing is applicable to Mac OS. (Essential tie, though Mac OS apps got a head start on this.) There are some poorly maintained apps that have subtly out-of-place elements, which is never the case on Mac OS. (Advantage Mac OS) Fonts/non-image UI is rendered at the exact effective resolution of the monitor at all times, compared to Mac OS, where you can only run at 200% DPI scaling for pixel-perfect scaling, otherwise the image is rendered much larger than the resolution and scaled down - this allows for the Mac OS advantage for the previous point, but results in subtle non-pixel-perfect scaling. (Advantage Windows)