I disagree that it's a small design change. The decision to use finger input as opposed to stylus input has a cascading effect on the whole UI. Fingers are too fat to click on lots of little buttons, so you have to use direct manipulation everywhere you can.
Take something simple like zooming or scrolling a page. In Windows Mobile circa 2006, the basic paradigm was the same one you'd use on the Xerox Alto in 1973: click on scroll up/down buttons to scroll, click on a menu and then on zoom in/out to zoom. The stylus was just a less capable mouse on a smaller screen. On iOS circa 2007, you pinch to zoom the page and swipe to scroll it. There is no analogue to this in the previous Windows/Menus/Icons/Pointers paradigm.
I think the iOS UI is really the perfect example of something that wasn't just tweaking. Windows Mobile was just a tweak of the mouse-based Windows UI for stylus input. iOS threw away the Mac UI, predicated on the mouse, and started from scratch. "What does a UI look like when you start from the assumption that you're manipulating it with your fingers?"