Not saying any role is "hard" in wow. But you cant argue that some are more difficult than others.
Here is my vote,
Tanking - Holding threat is not difficult really. Unelss your undergeared. But tanking Id say is the leader of the group. You have to know every fight/encounter, how to place the boss, and Id say it was the "hardest" role to play end game. If you fail as a tank, the group fails.
Healing - Healing is fun, its what I do mostly. Id say it comes in second place. Healing end game, does require some skill. I know its a bit of whackamole, but in encounters like LK/sindra, you have to know what your doing. You have to know to freedom the tank on sindragosa so he can hurry behind the ice blocks, you have to time dispells on LK appropriately, while watching all the meters and standing out of green sh*t/fire.
DPS - Id say dps gets the easiest role. You can fail as a dps, and still get carried VERY easily. much more forgiving. you learn your rotation, stay out of crap on the ground, and know what to pew pew.