Complete ground-up re-design from previous Dura-Ace front derailleurs. Much stiffer cage mated with heavier springs now make front shifting feel as quick as the rear. XTR gray anodized finish with hints of polished alloy are absolutely beautiful. One caveat, the 7900 front derailleur only works with 7900 STI levers.
The nitty gritty - the spring tension is optimized to provide dramatically smoother shifting verses the solid but slightly clunky 7800. The linkage design is also improved to reduce friction and provide a noticeably lighter shifting action. Most significantly, no trimming is necessary. This means you can shift through the whole range of cogs in the rear and not have to worry about trimming the front derailleur. Lastly, Shimano has reduced the weight of the 7900 front derailleur by 7 grams over the 7800 model.
For info on 7800-7900 compatibility, see the Shimano Dura-Ace 7900 Compatibility Chart