The steambox looks like a PC actually. Without the advantages of a PC. I don't know what to think about it. With the current building it would hardly cost under 400 euros. A Xbox, PS3, or the Wii (and certainly Wii U) are way less expensives.
Valve, with Steam platform, is an eldorado for indepedant developpers. Because the Big Three owners (Microsoft, Sony and Nintendo) won't give them easily a "sesame" to launch their game/project on their own platform. So, the steambox would allow you to access to a LOT of indepedant games, at very low prices. The thing is, with such hardware in its box, it is also designed for big games. I don't know if it is interesting for console gamers (casual or not), you can have all of this in a PC. You can also configure a PC for your tv.
There is also the problem of the distribution. Will there be many physical distributors who would agree to sell console which only offer dematerialized games ? Of course Valve can decide to sell it online only, but will it be able to reach a big audience ? All the console players are not hardcore gamers.
Another point, since the Steam Box is basically a PC tuned, there is the problem of Direct X. Almost, if not all, all PC games need that to work, and Microsoft is its owner. How do you think Microsoft would react ?
... But I played SF X Tekken online and got my butt kicked, so I need more practice... Like Colonel Hannibal said in 'The A-Team', Give me a day, I'm good. Give me a week, I'm great. Give me a month, I'm unbeatable......So I'll fight you in a month 
!
I also have to restart Lost Odyssey, stopped at the beginning of the second DVD, when the old 360 red ringed.