Agree with Steppy on the plastics quality, particularly interior door trims which are a real downturn in perceived quality feel compared to Audi of old. New A6 is similarly less impressive on the door trim front and I was disappointed at this when I was considering what to replace my A6 with last year.Quality of plastics is definitely lesser; door cards are much more lightweight, trim surrounding heater control plastics don't have the nice soft coating etc. I know that's mainly cosmetic, but the build quality on mine is definitely no where near as good as my A3.
The dash itself is great, no problems there at all, but my main complaint are the numerous creaks and clicks from the rear of the car. I have wedges of foam between the rear quarter light windows and the trim which supports the parcel shelf to stop creaking. Both the A pillar trims click and crack over bumps when the weather gets warm and I get noise from the rear door cards when playing certain types of music - not very B&O. I only had the basic system previously but I could crank that right up and I didn't get a single vibration from the door cards.
Having said all the above I still love the car, just disappointing considering the cost involved