I feel like the "casual 15 mins gaming" is predominantly going to be iPad and cell phones in the future, whereas I see SteamOS fitting the "I'm going to spend two hours in my man cave" scenario. If it's a box in your living room, presumably you could trigger an installation remotely from your computer or cell phone, and when it's done downloading an hour later you can just walk over to it and start using it.
It's the barrier of entry of having to download and use up all that hard-drive space. It's like having to install an .exe vs. visiting a website. Even if you trigger the .exe installation remotely, it's just not as convenient and agile as visiting a site at will.
