My biggest reason I don't tank is honestly because I do find it boring. I don't like the combo system as it both feels constrictive and it feels to slow. I like the NIN and MNK combat styles since their speed buffs to actions makes the actions feel more flowing, much like the casters where usually you can cast one spell after another as each one finishes casting. You don't have this awkward pause between actions like you do with the three current tanks due to the instant attacks being limited by the GCD.