I think he's so good at designing successful games because he doesn't like them so much. Unlike everyone else in the industry, he doesn't seem to play games that much.
Most developers join the videogame industry because they love videogames and they want to make their videogames. Videogames are both their work and leisure.
Miyamoto was just some guy who was working at a company who branched out. He doesn't really want to make his games, he just wants to make games... and since he isn't making them for himself... he's making them for everybody else.
He often gets an idea or concept for a game, and then his development process appears to be to develop stuff, bring in outsiders and force them to play, and find out anything and everything that doesn't work and change it.
Since he isn't particularly attached to any of it, there are now sacred cows. Now it can't all be that obviously, no amount of changing is going to turn a piece of crap into gold... but I think this is a BIG part of why he is behind so many 10 million sellers instead of 2-3 million sellers.
Now... what happens if Miyamoto has a game that he wants to make? A game, like most developers that is "His" game?
Miyamoto... an avid musicisan REALLY wanted to make Wii music. He really likes playing guitar.
http://www.gamespot.com/users/bretikur/video_player?id=cnw7lDSr5bgOujfb
It was a product he really got attached to... and well ok it still has sold nearly 3 million copies, but it's less then they thought it would sell.
I think it's his weird combination of being a creative person who is somewhat detached from his work, that makes it so easy for him to create MASSIVE sellers. Sort of a Zen designing process where everything is forgotten except what you get from expiermenting with players.
