In the leaked screenshots of Office 15 you will notice the ribbon is minimized by default and using a full screen menu, it can brought back using a pin icon and can be minimized with the same.
Another interesting feature is “Present Online”, this allows users to share Office 15 documents online using Windows Live ID. Broadcast Slideshow feature in PowerPoint 2010 is renamed as Present Online and expanded to other Office applications in Office 15.
Office 15 first public beta is expected to start this summer, we can see if there are any new features and UI changes to Office 15 in those builds.
Source for Images: The Verge