Some people describe a car to be fun to drive when the performance of the vehicle matches their preferences. That is why you have a segment that enjoys Ford Mustangs for their straight-line acceleration (and calls them fun to drive), and you also have another segment that enjoys the Mazda Miata for its sport-tuned suspension, short tranmission gearing (which allows the engine to stay on the powerband with less effort), and low weight (and also calls them fun to drive).
There are a thousand different ways that a car can be fun to drive. Some are not even related in any way to the actual car, but to the sentimental value that the owner puts on the vehicle.
The most fun to drive car I've ever had was a sub-compact that barely had eighty-eight horsepower. Yet it did not consume much fuel. Such attibute pushed me to find how far I could go before needing to refuel. That alone made the car fun to drive.
The Mercedes owner above values the rich set of features that his vehicle has. He seems to enjoy knowing that his vehicle is highly equipped to whithstand and accident, and is even able to reduce the chances of one actually happen. Plus I bet he loves driving past store windows, and see himself driving such a fine automobile. I know I would. =)
I think you meant subjective ;)
The more languages one speaks, the less one speaks one.