Microsoft presented itself an enormous task when it started development of Windows 8: Produce an operating system that could go toe-to-toe with iOS and Android in the touch-driven tablet space while still preserving the value of Windows' considerable history on the desktop.
This complex problem has challenged Microsoft for about two decades. For 20 years the company tried to ship touch-driven (generally stylus-driven) tablets that used the conventional Windows front-end, hoping a smattering of extra utilities—an on-screen keyboard that floated over the Windows UI, boxes to contain handwriting—would be enough to convert a mouse-driven operating system into a tablet platform.
These tablet machines never achieved substantial success. Apple's iOS, first released in 2007, demonstrated that touch machines could find enormous success as long as their interface was sympathetic to both the constraints imposed by touch—imprecision, obstruction of the view—and its novel capabilities. Before Windows 7 was finished (and before Apple had even announced plans to produce an iOS-running tablet) Microsoft started work on developing such a user interface for Windows.