The Dura-Ace 7900 cassette features an updated tooth design for optimal shifting and minimal friction when used with the 7900 chain. The largest four cogs are made from titanium. A new aluminum carrier contributes to a 10 gram weight savings over the 7800 cassette. As a bonus, Shimano incorporates increases in rigidity in the new carrier.
Improved performance, lighter weight, and increased rigidity are nice accomplishments, but even greater is the wide range of ratio choices--eight to be exact. Flatlanders and time trial specialist can still have their 11-21, but now an 11-25, 11-27, and 11-28 are offered so you can go up and down the hills with ease. The additional cassette ratios are also very handy when used with the new compact crank to provide a wide range of useable gearing for any riding condition. The 7900 cassette is compatible with the 7800 rear derailleur and chain, however, the 11-27 and 11-28 require the 7900 rear derailleur.
For info on 7800-7900 compatibility, see the Shimano Dura-Ace 7900 Compatibility Chart