During an on-screen demonstration of Windows 8 touch features at Microsoft TechEd 2012, Microsoft showed a glimpse of the portable device (Windows RT) edition of Office 2013, specifically Excel. When compared with a desktop copy of Office 2013, some changes come to light. (The Office "15" preview I'm running now is on top in each image.)
A Wild Preview Label Appears
The copy of Excel shown on-screen was labeled a "Preview Edition", in line with expectations that an Office 2013 Beta is coming this Summer.
Where's the Ribbon Pin?
Office "15" Technical Preview offers the ability to permanently "pin" the Ribbon -- that is, keep it permanently expanded. In the "RT" sneak peek, however, it appears the pin isn't available. I suspect the pin intelligently hid itself, perhaps detecting the device was running at a lower resolution. You can definitely see how letting users pin the Ribbon would lead to a squished worksheet with 4 rows to play with.
More Tweaks/Fit n' Finish
It should come as no surprise that Microsoft is still working on Office 2013. For the sake of completion, I highlighted the replacement New Worksheet icon and highlighted a scrollbar track color change that, in my opinion, looks much better.