If you're not familiar with the thinking behind Shadow, here's a recap. The idea is to have a lower-profile mechanism that doesn't stick out so much (up to 18mm narrower than a conventional derailleur) and is hence less vulnerable to rock/root/falling over damage. It's also said to be stiffer thanks to generally shorter pieces linking the parts together and has shorter, more direct cable routing.
The forthcoming XTR (RD-M972) mech (available in September) uses an X-shaped outer plate and full carbon fibre cage to make it the lightest derailleur in the Shimano range at a claimed 180g. Short and long-cage versions will be available, and the unit will be top-normal (or "conventional" as we like to think of it) only for use with Rapidfire Plus shifters.