Tanking is, hands down, the hardest role in a group, both technicly and in the dynamic of a group. Its not hard when compared to other things [DMC is hard, if you want a measuring tool], but in the context of the game, its the hardsest it gets. The reason is that you are basicly incharge of the group, and you have to make sure none of the stupid things the other 4 people in the group dont kill you. you are there to fix the problems the other people in the group create with their lack of basic skill.
Regular pull? You have to mark down the targets, pull'em, and make hold them all, making sure no-one escapes you attantion.
Mage pulling aggro? Now you have to grab the target and make sure it doesn't run again, while the mage blinks around the room, pinballing the mob.
CC brakes? You have to grab the target and still hold the main target.
People attacking random targets? Grats, now you get to run around and wish you can grab someof the mobs while your team basicly does wahtever they want.
And so on and so on. Even when the team works perfactly, you have to be the best of the group, to make sure they all get to do thier job without dying.
And in the realm of tanking, the warriors have it best, or worst, depanding on how you look at it. They have the most abilities, which means they have a tool for every situation. On the other hand, thats alot to master, and you can easly get lost in it. even when it works, if you have two problems you need to take care of, the GCD keeps you from doing it, because you can only do so much within a time limit.
So, yes, its hardest to tank, and if you tank, its hardest if you are a warrior. Its not impossiable, and its really fun when it works, but when it doesn't work, watching your group run aimlessly, like headless chickens, even after you worked out a good plan, and nothing bad was supposed to happen, is really frusrating.
Tanking as a warrior is the part of the game where that ESRB warning about gameplay and muliyplayer is truest: Its awesome in a good group, and i know some guy who used to have hair before he tanked a bad group.