I wonder how upgradeable the steam box will be, the size seems to point so design changes such as custom cooling solutions and overall the thermal limits, (eg upgradeable GPU's on the old alienware laptops failed because it was difficult to get new and faster GPU's that match the thermal limits of the old GPU, and thus, upgrades were minute and expensive.
For the steam box to be successful, I should be able to buy any component that is upgradeable from a normal computer parts store, eg I should be able to go to newegg and buy a GTX 770 and install it into the steam box and have it work.
Other than that, I can see the steam box being used for convenient PC gaming for novice users. for example, if there are different tiers of steam box (allowing for different price points but same or similar hardware at each price point, then steam can have special profiles (kinda like how the crappy nvidia geforce experience works, but actually work correctly) . Steam can test each game and go through all of the settings and from a humans perspective, create custom settings that balances the best looks with acceptable frame rates. (most game presets now will not balance the settings properly (eg on limited hardware, on some games, the medium preset may offer 50FPS but look like crap, but a user using a mixture of low, medium, and high settings will maintain the same frame rate, but have the game look much better where it counts.)