If it's a closed hardware system then it's just another console. I don't need a third (or fourth if Nintendo gets their act together with the WiiU and convinces me to buy one) system like that.
I do wonder about that myself. On one hand, creating a closed system makes it more comfortable for a console gamer. It would allow Valve to create a hybrid system that appeals to a console gamer without needing the technological know-how that is generally required of a PC gamer. They can release a PC prepackaged with all the hardware necessary to run today's games and ensure people that will be good enough until Steam Box 2, which is the sort of comfort level I think console gamers like, particularly among the younger players.
Oddly enough, that's sort of what Microsoft's goal was with GFW Live/Vista a few years ago. They wanted every system and game to have a score, and then you could just look on a box and easily tell if your PC could run the game or not. Microsoft approached it from a software perspective, whereas it sounds like Valve will be taking the smarter approach of a hardware perspective.
On the other hand, why not give people the option to upgrade if they wish to? Make it the best of both words: People who don't know much about computers can avoid upgrading if they wish, and those that do can maximize their system to their heart's content. Potentially, Valve could even streamline this each year by branding various hardware upgrades as "Steam Box Upgrades" or whatever. Again, those that know enough about PCs can shop around and do their own research, while others can just buy whatever Valve says will work.
Though, I still wonder if that'd alienate certain people. Even if you can easily recognize which videocard is an upgrade necessary to play most games on the market, I'd guess a lot of people wouldn't be comfortable opening their system and trying to figure out how to switch it out. Ideally, a company trying to win over that audience should create a system that is very user-friendly when it comes to installing upgrades (like just being able to slide out the videocard and slide in a new one and be done in a couple minutes), but that'd take a much larger commitment from the hardware industry. And that gets back to why I could very easily see this being a closed system. After all, Valve already won us PC gamers over years ago. I don't think it's us that they are gearing this towards.
