The ability to run Mac OS X apps on the iPad, with full access to the file system, peripherals, etc., would make the iPad worse, not better. The iPad succeeds because it has eliminated complexity, not because it has covered up the complexity of the Mac with a touch-based “shell”. iOS’s lack of backward compatibility with any existing software means that all apps for iOS are written specifically for iOS.
It’s the lesson that so many hardware and platform vendors can’t seem to learn: the pretty skin isn’t why people like Apple products, and applying a pretty skin over your products won’t make them compete better with Apple’s.
Steve Jobs said it best: “[Design is] not just what it looks like and feels like. Design is how it works.”