The new Dura-Ace front derailleur is upgraded with significant improvements. While the STI levers and crank may be getting the most attention, the new front derailleur might contribute just as much to improved shifting performance.
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.