This new generation of digital SLR almost doubles resolution with its new eight megapixel CMOS sensor, doubles the size of the continuous shooting buffer. Despite the advertised 8.5 fps the EOS-1D Mark II actually shoots at a maximum of 8.3 fps (the same as the EOS-1D).
The EOS-1D Mark II is also obviously Canon's answer to Nikon's fast shooting D2H announced last year, it does appear however as though Canon are raising the bar with resolution and one would hope we can expect similar low noise performance from its CMOS sensor as we have seen from the same technology in the EOS 10D.
Putting the additional resolution and buffer size to one side there are also a variety of improvements and changes compared to the EOS-1D, not least of which a new version of Canon's DIGIC image processor, an extension of selectable ISO sensitivity, support for Secure Digital as well as Compact Flash and the addition of a USB port (although only USB 1.1). The SD slot and USB connector are interesting additions as they may hint towards potential future add-ons for this camera such as a wireless kit (pure speculation on my part at this stage).