Seamlessness and the move to Windows 8
Multi-touch is hardly a make or break feature in and of itself otherwise the HP TouchSmart would have captured the PC desktop market. HP had to deliver a lot of custom apps to provide a reasonable user experience. The key to Apple's success is the relative seamlessness of support across popular applications.
For their part, Microsoft made touch UI game-changing moves starting with Office 2007's Ribbon and Windows Phone 7. My opinion is that Windows 8 converges these and will provide a reasonable and more consistent user experience across a wide spectrum of devices. Whether users used to iPhones and now iPads like that experience enough to run it as their primary PC OS has yet to be seen.
