The Dura-Ace 7900 rear derailleur is a complete rework from all other previous designs. From first glance you notice the distinctive XTR-like anodized/polished finish and carbon cage, but the big changes come when you take a look under the hood. New pivot points and longer cage accommodate a wider range of cassettes(up to 28 teeth with a compact crankset). Heavier return spring offers quicker action and alloy pins sandwiched between the plates add rigidity.
The 7900 is 100% compatible with all other Shimano 10 speed systems and serves as OE replacement for the 7800 rear derailleur.