From Tom Warren at The Verge:
"Overall there's clearly a number of improvements and new features in Office 15, and Microsoft is only at the beginning of testing some of these externally. Microsoft isn't discussing any of its planned features for Office 15 just yet, but the features we have detailed today are part of its key testing phase for the initial Technical Preview. We expect to see some more improvements in the Touch Mode interface for Office 15 and perhaps some additional features in each client application once a public beta is made available this summer."
This looks like the natural evolution of Microsoft's Ribbon interface that will be touch-optimized. (Perhaps their iPad version, which hasn't been announced/released, will show some of the same functionality!) Microsoft moved to the Ribbon interface with Office 2007. However, Outlook wasn't really updated until 2010. Given this early preview, it looks like all Office applications will benefit from the enhancements this time.