There are many improvements in the Ultegra 6800 group and perhaps the most notable (along with the crankset) is the effortless, lightning-quick front shifting. The Ultegra FD-6800 front derailleur delivers a shorter stroke at the lever with a lighter action. To achieve this noteworthy reduction in throw and shifting effort, Shimano uses new pivots with a longer arm to the cable pinch bolt for increased leverage. The pinch bolt is a low-profile design and the cable now runs along the lower edge of pinch bolt.
We should mention others sources of improved shifting include Shimano's polymer coated cables for light and friction-free performance with no stickiness issues. Like Di2 front derailleurs the mechanical braze-on version of the FD-6800 has a new 2mm hex support bolt that stabilizes the front derailleur against the frame by providing another contact point. The FD-6800 has a trim feature so it can be fine-tuned to be quiet in all gear combinations - even if you cross chain from time to time. All of these details add up to practically seamless front shifting so you can get Di2-like performance in a mechanical system. Compatible with standard, compact and TT chainrings - just keep the large and small difference at 16 teeth or less.