All of the exterior black plastic trim (bumpers, side skirts, lower valences) are all gloss black now rather than the matt unpainted plastic of the MK8. Definitely a big visual up-lift with this alone imo.
The new headlights are quite subtlety different at a glance, but definitely are an improvement visually - The front end of the car which wasn't the MK8's strongest point is much improved imo.
Rear taillights have the latest LED's, similar to those in the latest ID cars, new Tiguan, Passat, etc - animation on unlock, etc.
Different and more attractive wheel options. The GTI now has a 19" wheel option (Queenstown) which is unique to the GTI (The Estorils are still an option but they're technically "R" wheels).
Interior wise, you've got the vastly improved infotainment system and physical buttons on the steering wheel. Some trim/finishes are different too, although some late MK8's may have this - GTI emblem on seats, gloss black center console, nicer material on door cards, different "carbon effect" trims.