I'm not sure I follow. Does Steambox offer a replacement to mouse+keyboard that doesn't work on a regular PC? Is it not possible to connect a regular PC to a television?
What is the difference between a Steambox and a HTPC with the same hardware in it?
What does a console offer that a PC doesn't? A single ecosystem? A terrible control system? If you want to bottom line it like that, that's about it.
You need to follow the OS and the licensing. Microsoft is headed to a closed ecosystem, has always had outrageous licensing fees, and is in general a poor steward of any technology unless threatened. (Look how long it took them to make a modern browser until FireFox and then Chrome began to take market share)
The only reason they have been the leading platform for gaming is that PC's running MS Windows dominated the landscape. Consoles made major inroads into that market, but their hardware limitations, slow evolution, and lack of a viable controller for certain genre's of games, not to mention the high price of those games partly due to the necessity of physical media have prevented them from taking gaming away from the desktop. Many gamers I know would love to get away from Windows as a platform, but you play where the games are located and consoles today simply don't cut it.
Now you have Valve bringing a system that is open source sporting a free OS making the stewards the people who have driven a vast amount of the innovation in technology. Look at what Android did for mobile. While the box isn't going to be "One spec to rule them all, One spec to bind them", it will be limited enough to gain the benefits, especially since the different components appear to be limited to GPU (all Nvidia which has had the best drivers for their cards for years and years) and Intel CPU's. Since they do not vary wildly in their architecture, you really aren't going to have much in the way of compatibility issues with them. In fact, I don't think developers will see
any adaptations to their code to accommodate the differences. So you still keep the benefits of a limited environment, but you can upgrade and so aren't limited to producing games for hardware that is now 8(!) years old. While not proven yet to my satisfaction, the new controller designed for the Steambox appears to at least have the potential to replace the mouse/keyboard combo in a way that the Xbox and PS have not. Finally, we are ALL familiar with what Steam has done for the price of PC games. They drove GameStop out of the PC gaming market in their storefronts. When you can play your games on a console for $20 instead of $60, MS and Sony are going to see a sharp decline in the sale of their own systems. All in all it has the potential to bring the best of both worlds to the gaming community and perhaps if successful enough, finally really bridge the gap between PC and console gamers.