Info is definately lacking.
There is a huge amount of driver and API overhead on Windows. That is why consoles with their vastly underpowered hardware still manage decent performance - mainly it's the ability for direct memory access. A Windows Steam-box is going to have these same issues, so it will require more powerful (and expensive) hardware compared to the more traditional consoles. A Linux-bases gaming oriented OS could alleviate this some, but not enough.
The Steam library is predominately Windows-based. I can't foresee Steam-box being so proprietary that it effectively becomes it's own eco-system, as it would fragment the market into Xbox, PS, Nintendo, PC, and Steam-box. For any real success it must have access to the Steam library with little-to-no modification. That leaves a Windows-based Steam box and it's expensive hardware and OS license, or Linux-based through ports and Wine. Many games work with Wine (I played WoW through TBC via Wine), but few work perfectly, and many need specific Wine configurations and even Wine versions. It's one thing for a Linux user to deal with Wine issues such as fonts and installer work-arounds, but it's another for a Steam-box buyer to have to. Is Valve going to manage this? Or the developer? How large does the Steam-box user base need to be for developers to judge it worthwhile to make a Linux port or ensure Wine compatibility?
How about game design? Every console game is designed for a 10' foot UI and a controller. Will this lead to the consolfication of even more PC games? What about game genres? We all like fighting, sports and adventure games on consoles, but what about all the RTS and strategy games currently on Steam, and every other genre that doesn't lend itself to controller play? Are we going to have Steam-box targeted games and Steam-PC targeted games? How about online between between Steam-box and Steam-PC (i.e. separate servers due to different controls)?
What about hardware? An AMD apu is probably the most cost effective, but Trinity on Windows isn't really enough for 1080p play. Perhaps the next incarnation will be. If Steam-box is Linux-based, can they still use AMD hardware due to AMD/Linux driver issues? Would Intel/Nvidia be too expensive? What about hardware generations/revisions? Is there going to be any incentive to develop for anything more than the baseline Steam-box specs? In 3, 4, or 5 years are games still going to be targeting the 2013 level Steam-box hardware? I've got a HTPC that (with a discrete GPU) will probably be more powerful than any Steam-box. Can I just install the Steam-box OS/App while using my own hardware?
It seems to me that the power of Steam-box lies in the gaming library and seamless integration between Steam-box and Steam-PC. Playing the game on Steam-box, syncing to the cloud, and picking up where you left off on your PC would be great. I doubt Valve intends to take on Sony, MS, and Nintendo with a dedicated console that doesn't interact with Steam PC, but then again, maybe they will. Games could be labeled as compatible with PC, Mac, Steam-box, or Steam-play for all three. What's the incentive for using Steam-box over a gaming HTPC? Just price? Then how cheap does it need to be versus just building a Windows-based SFF PC that will play everything on Steam?
Obviously Valve has thought all this through and believe they have solutions. And they probably do. How about just some sort of extender for graphics and the controller that uses your current PC for the processing?