The feature I'm most excited for is the versatility of having the trackpad and keyboard pair over lightning. Not only can you charge them via lightning cable but they work over USB and you don't have to fiddle with Bluetooth settings to bring them over to another machine.
As a person who moves frequently between MacBook and iMac, this feature is huge. Beyond that, the addition of force touch to the trackpad is genius and I cannot wait to see what developers do with this new interaction method.