- Macros, add-ins, forms, ActiveX controls (PowerPoint), and custom programs.
- Send email features (since Outlook isn't part of the RT version of Office 2013).
- SkyDrive sync integration.
- Equation Editor.
- Lync file download.
- Legacy media or Flash video playback (PowerPoint).
- Integrated audio and video recording and search (although this could be performed outside of OneNote and then inserted into OneNote).
For full details on the Office RT product, take a look at the Office Home & Student RT Preview Page. See also a Windows RT discussion transcript of an open forum with the Windows RT product team--very interesting!
For help dealing with another Windows RT limitation, see the FAQ "Q: My Windows RT device supports an SD card but the music, videos, and pictures on the SD card can't be added to libraries--how can I add them?" And for more about the productivity aspects of Windows RT and Surface, see Paul Thurrott's "Taking Surface to Work."