Here’s a snippet from an excellent article by John Gruber in his Daring Fireball blog. The article is entitled Why Windows 8 is Fundamentally Flawed as a Response to the iPad. I suggest you read the whole thing.
But I think it’s a fundamentally flawed idea for Microsoft to build their next-generation OS and interface on top of the existing Windows. The idea is that you get the new stuff right alongside Windows as we know it. Microsoft is obviously trying to learn from Apple, but they clearly don’t understand why the iPad runs iOS, and not Mac OS X.
Microsoft’s demo video shows Excel — the full version of Excel for Windows — running alongside new touch-based apps. They can make buttons more “touch friendly” all they want, but they’ll never make Excel for Windows feel right on a touchscreen UI. Consider the differences between the iWork apps for the Mac and iPad. The iPad versions aren’t “touch friendly” versions of the Mac apps — they’re entirely new beasts designed and programmed from the ground up for the touchscreen and for the different rules and tradeoffs of the iOS interface (no explicit saving, no file system, ready to quit at a moment’s notice, no processing in the background, etc.).
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.
There’s a cost for this elimination of complexity and compatibility, of course, which is that the iPad is also less capable than a Mac.