About to say this SteamBox is just Steam, but in a hardware storage console. But before I get called out on being wrong that is what I envisioned, since Steam has many different third-party companies on its service.
Yeah most PC games get on Steam so it's not like you'll run out of good games to get on the SteamBox. But there are some other companies which most likely won't have their games on steam. Guess we won't see an OriginBox any time soon.