Here is a cropped screenshot of the ADK schematic(just the headers). The Due brings a lot of more efficient and powerful features and if the pins are not brought out to the headers then new shields will not be an option. https://docs.google.com/open?id=0B80-mYIKLYFrQW9xNUU4RVc3Z00
Two main feature are Ethernet and SD cards. They both have dedicated hardware so they don't have to share the spi bus and will have better interfaces. It looks like all of the pins(B0-B9) are brought on the dual row header for a new ethernet shield. But the HSMCI pins are not on any of the headers for the SD card interface. The ADK does have a micro-sd card slot already onboard though.