As described in this thread the steam of has no market. I fail to see how it is different from a PC.
It is a non-competitively priced console with a very short lifespan for those looking to play the newest titles. It is no different than if MS came out with a backward compatible successor to the Xbox every 3 years except charge more for it.
Steam has no market? It is it's own market! It has it's own delivery system, etc. etc.
The only hurdle Steambox would have, is getting games to run on a non-windows based platform. That's it.
It's a pretty big hurdle though.
A steam box would be as different to a PC as an XBOX360 is different to a PC. They are pretty much the same