tanking was the hardest thing in wow vanilla, was hard in tbc, got terribly easy in wotlk.
however (maybe i'm going to write something that someone already wrote) i can say that tanking, healing and dps may vary in difficult depending on the class.
i've played several years different classes, playing only mage, pala and warrior in cataclysm and at the moment i can say that:
Warrior: Tanking is hard, DPS is average in crowded situations but is very pleasing to play when you find yourself managing adds and trying to save the ass to your healers/clothies.
Paladin: Tanking is boringly easy, Healing is boringly easy if the tank is good and the DPSes knows how to avoid damage, i cannot tell about the DPS becouse the current retadin is heavily RNG dependant.
Mage: Arcane is just 3 button spam, Fire is more fun than Arcane but still mindless, don't know about Frost, i dropped that spec since lvl 68.