Example. If you are tanking you have to taunt off of the npcs in ur party and prevent the damage from going everywhere.
Healing is obvious and dps would need to use cc's to complete the hard quests successfully.
I believe this would train players to excel in their given roles when reaching at level content with their toon. It would also allow a feature that gives 2-3 players the ability to quest together within their roles and gain experience.
If you agree with this idea and or have suggestions. Please leave a comment.