An upgradable machine also allows a "power" user to upgrade their machine way beyond the original specs in order to give them an unfair advantage in online gaming. I picture someone with a monster 24 CPU/256GB RAM box playing TF2 at 200fps pummeling me ever harder than they do today. :(

However, I like the idea of a Valve console. They've done so much right in the world of gaming that I'd like to see how else they can shake it up. :)