Quality, quality, and more quality.
When I worked for BMWNA I learned a LOT about these cars. Just a small example; RR has their own cattle heard for the leather interior. The fencing around the pasture is designed to not leave a single mark on the hide. Every square inch of the hides are meticulously inspected for flaws prior to being allowed to be in the car. If there is a single imperfection, the hide is not allowed to be used.
They are incredible vehicles. Seriously.
-Marc