Then the class balanced needs to be based around the concept that "being good" = being able to think strategically, being able to adapt dynamically to different situations, being able to think quickly, being able to coordinate and cooperate efficiently as a party, being able to put together a good combination of gear and skills, and understanding well the gameplay mechanics.

There's somewhat of a fine line between that and "exploit", but that's all subject to balance changes like every other game out there. It's hard, but not impossible.