Have you read Gladwell's book "Outliers"? Basically deals with the situational factors that create outliers like Bill Gates. Circumstances have incredible life-determining effect that go totally unnoticed too.
For me, it has everything to do with avoiding pain points on the consumers end. Keeping a consistant design/feel means Apple doesn't have to teach consumers new skills. Other platforms have to differentiate with size, shape, and OS. Each one requeres a transitional learning period.
Take siri for example. Apple is running a ton of celebrity ads just to show people how to use it, because siri is too different for the average person. Luckily for Apple, its just a feature, not the core product. Just like a new Android device, people will get used to siri eventually. The difference is the iPhone consistency makes the essential phone experience seamless.
One of the big things that Apple got amazingly right was the App Store install process. From a Linux perspective it's mostly an evolution, but for the mainstream customer that has had to deal with Windows installers it's unreal how easy it is.
I saw this link in one of the comments in the microsoft.com design preview post:
It's worth pointing out that the app store is one of the things apple didn't get right 5 years ago. It came later, after they'd spent almost a year claiming that the proper software extension mechanism was web apps. Obviously that doesn't detract from its success, but I think it's an important part of history. The app ecosystem that we all associate with "iOS" beat Android to market by only about 6 months.
Cell phone operators had "java stores" that were just as easy to operate (and would even just show up on your monthly bill, so no credit card needed). It's just the iPhone was the first platform good enough that people wanted to buy apps for.
