Clay is all about grunt, power and running around all day. Its the slowest surface so ball sits up for an age. Its a big leveller which helps the plodders. Faster surfaces of grass and hardcourts provide the real examination of tennis ability.
A lot of the all time greats of the sport never won the French Open - Sir Andy Murray, John McEnroe, Jimmy Connors, Boris Becker, Stefan Edberg, Pete Sampras.
Have a look at the French Open Roll of honour and its littered with no marks who never got within an asses roar of winning a Grand Slam on any of the other surfaces - Andres Gomez, Sergi Brugera, Thomas Muster, Alberto Costa, Carlos Moya, Gaston Gaudio.