I love this idea, and I was really expecting concepts like this when I first heard about Steam boxes. Computers have changed dramatically in the 20 years I've used them, and things keep getting consolidated. There are about 5 important parts now to the average gaming computer, and I have no idea why we're still dealing with circuit boards for something that's essentially plug and play already.
I'm sure everyone in this thread can build a computer and wouldn't be intimidated by it, but there are advantages to this that go way beyond an idiot proof build. Imagine how easy it would be to pass your older computer parts to family/friends without installing it for them, or maybe traveling to their house for the holidays and just bringing a module or two you need instead of a laptop. People mentioned it earlier, but troubleshooting would be another huge advantage.
True story:
A chick I work with bought a computer a few months back, and even though she doesn't actually know a lot about computers, she was able to build it herself. She wasn't happy with the Intel HD graphics though, so she went out and bought a graphics card and a new PSU afterward. (On that note, the 700w PSU she bought was way more than what she needed, so it would be really nice if these modules included some kind easy to read power rating.)
Anyway, that was when the problem started. She said it made an annoying "whine," but only when she was playing games, and only when she'd been playing for a while. Sounded to me like the new graphics card was just getting hot and the fan was too loud. Someone else took a look at her computer, and somehow confirmed it was the fan on the graphics card, so she bought a new fan and installed it herself.
The problem continued though, so I tried taking a look at it. Now we think it's actually the PSU that she bought at the same time, but it would be a total pain to swap our power supplies just to test that. Next to a motherboard itself, a power supply is probably the most annoying thing to swap out. It really shouldn't be more complicated than buying a new power brick.