The main thing I want from Steam OS is the standards it brings about. Not so much for controllers (the Xbox 360 pad already did most of that), but for other elements, many of which are intangibles. We'll see on the controller front tho. However, Steam OS in and of itself doesn't really provide any new features you don't get via Steam on Windows. Every new feature is there, streaming, music, Big Picture mode, all of it.
What does seem better on Steam OS is the tighter/smoother overall integration of the OS. But that's what happens when you can control all the elements of the equation.
Qassim sums it up well:
On Windows, the Big Picture experience can still be a bit janky. It's not Valve's fault, they have to work within the limitations of Windows. But Steam OS allows them to tailor so much of the experience to playing games, that it all just seems to flow better. Transitioning into and out of games is perfectly smooth, the notification system is really quite cool, and the fact it is built from the ground up to be used with a controller is exactly what is required for a living room based system. After playing with Steam OS, it became immediately apparent why this would bring advantages to gamers, maybe not now, maybe not even in 3 or 4 years, but the potential is there to bring something that Windows cannot.
I think Steam OS is a long-term project, not one that will be their dominant system in the very near future, but one that will slowly mature until one day it becomes a significant portion of the Steam user base.
I think it's up to Valve to address these issues in Steam for Windows as best they can, and not leave them to stagnate to then point out in unfair comparisons, like they did with OpenGL vs DirectX when they first went down this route.
What I mean by that is they compared a pretty much untouched version of whatever game running on DirectX (9?) vs a newly updated OpenGL build, and almost no one even bothered to mention they also did the same for Windows, (which was much further down in the article) and the performance gains were about the same there.
I personally care very little about using Steam OS in and of itself, and I think it'll be relegated to people who want to get into PC gaming for the better graphics, mods, everything else that PC gaming can provide, but without the complication. Something that's console-simple.
Everyone else will just run Steam on their OS of choice. Which too is a good point, Mac OS and Linux users are getting more games than ever. Not so much because of Steam OS, but because the whole movement isn't just about the OS or the controller or Steam Machines, but also about running games on more than just Windows. I personally don't care about that either, but
"a rising tide lifts all boats".