I have to ask:
Who does it appeal to?
What features does it add over the competition?
What drawbacks does it have?
1) Current PC owners who want to play in their living rooms. Well this is a crowd which can run a HDMI cable to their TV and plug in a wireless controller or kb/m and get this experience right now. For those that cannot do that for whatever reason (cabling etc) then streaming is always an option. But that doesn't require an OS it just requires a small little streaming device kind of like what OnLive uses.
2) People who want a HTPC but don't already have a good PC, well it makes sense to build a HTPC or buy a prebuilt one, but if you install SteamOS then you're running a custom distro of linux and the games you can play is going to be slashed down to next to nothing. A better alternative is to simply buy a console, they're cheap and offer all the big name titles.
What does custom linux offer, customization? Well what exactly is it about windows that so badly need customizing? Less input lag..er what, windows has no percieveable input lag, there's way more in bad devices than in the OS (slow monitors, slow mice etc), performance optimizations? I'll believe that when I see it, but right now the fact is this makes next to no difference, due to consolization a slump in PC gaming has meant that we can run almost all of the latest games at 1080p easily with even mid range hardware, a bump in speed really means nothing. For anyone currently running a gaming rig a new piece of hardware would be a better investment than building a steam box, so that doesn't make sense. Open-ness? Why does that matter, how does that benefit gamers? How will a valve-controlled OS be any better than a MS controlled OS?
I love Steam and Valves work in general, but I literally cannot see the point of SteamOS and dedicated hardware, I think those looking for simple to use living room gaming will go to consoles and people after a PC will be better off simply cabling a decent gaming rig to their TV, I do that to my projector with a single HDMI cable and use my PS3 controllers via bluetooth with my PC, works like a charm, cost a few quid for the HDMI cable...