Most likely advanced PDF support, a Windows CE or so, with support for USB devices, audio and video playback ability, reading of .doc, .txt, and perhaps even .avi when it would have PixelQi's screen.
If it has a basic E-ink screen, it would need to have some sort of webcam, audio and external monitor connection to do something with the extra computing power.
But it would be cool if it could also support compressed text and html (.zip/.bz/.rar or .7z/.tar). That would be awesome!