My biggest problem with the idea of a SteamOS for gaming is that it won't include all PC games. With Steam running in Windows, I can run any PC game. ANY GAME. If it's not on Steam, I can add a shortcut in Steam and still run it from there. I run Blood from a Steam shortcut that launches the game in DOSBox. Will that work in SteamOS?
Then there's also the loss of other Windows programs. Most of my PC gaming happens with MMOs, so I often alt-tab to an IM program, one of three voice chat programs, a media player, a web browser, Notepad for taking notes, sometimes even Calculator. When I'm really into an MMO, I run a Ventrilo and connect to a server before I even start the game, and can chat with friends while waiting for servers to come up or whatever. With SteamOS I'm assuming we'll only have a web browser and whatever in-game voice chat Steam offers. Running in a dedicated Steam environment will likely limit my machine's functionality by a LOT. Oh yea, and just how many people are going to want to dual boot? I don't want to, and the average Joe who got a computer for school and then got addicted to WoW or DOTA won't want to. It would be way more convenient to have a Steambox sitting next to a Windows PC on the same desk, but then it's just another machine to deal with. If every single game that came out was on Steam, you could avoid the need to upgrade the Windows PC, but that's not going to happen. Most MMOs don't end up on Steam, and there's Origin. Want to play Battlefield 4? I doubt you'll be playing it on SteamOS.
The main purpose I can see for SteamOS is for the people that want the absolute best graphics and customization (PC), but really want to play it in their living room from their couch. A lot of these people are already doing this with Big Picture Mode, and SteamOS will make it easier and add some functionality. The controller will give the option to sit with a controller in your hand and play mouse-based games (though less effectively than if you used an actual mouse. I really don't see anyone playing something like DOTA2 with this controller.) If that's the role of this new OS though, is it worth the time and effort Valve is putting into it? Or is it just going to splinter the PC into 2 platforms? We already have to choose between Sony/Microsoft/PC when looking at where games are, is SteamOS going to be a new fourth? Will there ever be games that require SteamOS and won't run in Windows?
EDIT: Oops, I forgot that Nintendo exists, lol. I guess that would make Steambox a possible 5th platform, not 4th. (excluding handhelds)