One thing I have seen mentioned here is input devices. I personally use a PS3 remote and a bluetooth dongle. No aiming required and brings a real media player feel to the experience.
The only reason for a remote, is a player of your choice is accessible with a button, no shortcut pointing and clicking. A wireless keyboard (as more bulky as it is) can serve as playback controlling, with a bonus for typing when needed (PC after all)
And with a (optimally) trackball (sadly there is one modern, in my opinion, viable option in Logitech) you can do first person anything on a large screen if anyone does...
play games as well windows 8.1 is a great alternative. Its honestly been the easiest to set up by far for any peripherals I use.
Though if you use steam at all and your games support Linux. Making a steam box with XBMC on it may be the ultimate solution. Only time will tell on that front though.
However, even in big picture mode, I do still use the classic mode on my HTPC. Reasons being, it is greatly laggy in transition. I feel menu options are a bit bigger than necessary, and there isn't any way to scale to taste. Steam OS, in my opinion, wouldn't be any different than a tweaked Big Picture mode. Then again, Valve and Steam had ALWAYS been bad in allowing their interface to scale, being stuck in ONE option.
Another off topic complaint, touch finger scrolling with their UI in their client window, does not work. It is as if Valve is throwing a hissy fit and hinting at something about not conforming to another input option, yet go a bit forward to Xbox360/PS3 years for controller options? I know many are giving gripes to Valve for the molasses manner in dealing with Half Life, but my gripe is also here with their UI, interface, and options with such.