Valve are not making a console. They are making a PC that is built to be more accessible and more appropriate for your living room.
The idea of making a "Steam box exclusive game" makes absolutely zero sense, if it is on the Steam box, it is on a PC, too. Gabe has said the Steam box will not be a closed system, you will be able to install Windows on it if you wish (the Steam box that Valve will be producing, at least).
The actual Steam box will not be as cheap as a console, it will not be a direct competitor to the traditional consoles, it will be a step to making the PC platform (Linux, Windows, OSX) more accessible and more attractive to console gamers, it'll be on your TV (if you want it to be), it'll push controller support for games and it already has a TV interface for use with a controller. The Steam box is about bringing the good things about PC gaming and the good things about console gaming together.
Realistically because it is still a PC, it won't do some things as well as a console, it'll still have some PC issues, but I imagine Valve are hoping they can get the experience close enough and combined with the benefits of PC gaming so that many will be willing to overlook those shortcomings.
I guess what I am trying to reiterate is that I feel people are expecting something that Valve are not making, you know they're not making it based on what Gabe Newell is saying, based on the fact they're open that they're getting into deals with many hardware partners, the fact they're intending for this to just provide a blueprint for other manufacturers to make their own Steam boxes, for that to work it has to work on standard PC operating systems like Windows and Linux.
I'm a bit tired of people suggesting that Valve will make one of their games "Steam box" exclusive to push adoption of it, you know? I'm sure Valve want it to be successful, but their aim is NOT to make a single platform out of the Steam box, it will be the same platform it is now on Windows, Linux and OSX. It'll just be another way to access that platform, it'll make that platform some sort of competition for the consoles, but I feel it will still be a thing mostly for PC gamers and console only enthusiasts, it will not be a mainstream competitor to Sony, Microsoft or Nintendo.
This approach is reiterated by Valve's "Good, Better and Best" approach. They're all aimed at just repurposing and moulding the PC platform in which Steam runs on to fit into your living room. This is not a console. I'll repeat, this is a PC. Don't get carried away.