^^This is very true.
You're mad!
What other PvE role do you
* Not only have to have perfect awareness of all the buffs, casts and procs available on you AND your target? But also,
* Have to have a strong understanding of key class pulling, threat and recovery spells in your arsenal, especially taunting between tanks?
* Have to be aware of not only every mechanic in every fight as well as the mechanics of specific kinds of monsters in order to determine whether to spell reflect, apply or not apply specific debuffs, or ask for the application of CC, what CC to use and when it needs to break?
* Have to have perfect awareness of not only new adds but also their relative distance and the timing of when and how to pick up the adds without causing a raid wipe?
* Have to have the self-awareness and confidence regarding your class to know when you already have far too much on your plate and dynamically assign tanking responsibilities to others? As MT, even if other tanks know what they are meant to be doing it, you still have to time transfers back and forth to perfection.
* Have to work your cooldowns in synergy with your healers to ensure that the damage you are taking is not creating a huge strain on their ability to keep you up but also whether its sapping other non-tank healers who feel forced to help and therefore risk causing others to die. Sometimes this is an acceptable loss, other times it is not - you have to decide dynamically?
* Have to know not only bog standard perfect-fight threat rotations but also how to intertwine survival cooldowns and threat when a boss becomes most dangerous (e.g. phase transitions and enrages when boss is almost gone)?
* Have to know how to kite, using LoS (line of sight) for pulls and raid damage avoidance, positioning (against cones, cleaves)?
* Have to have a high raid awareness knowing where all your healers are as the priority group to keep damage away from at almost all cost?
* Have to have a high raid awareness knowing where all your silly dps are, including ranged, to ensure that if someone unexpectedly pulls you're there ASAP to pick up?
* Have to have a great raid awareness (+confidence+control) to know that you are tactically in charge of the raid, not the other way around, and that if e.g. the raid gets too close to even one mob who can Fear or cause loss of control of the raid, and wipes, you will most likely be blamed?
* Have to understand many key interacting statistics in gearing your tanking spec and how they impact your gameplay, threat and survival: misses, dodge, parry, mitigation, block, armor, effective health, diminishing returns, what does strength give you, what does agility give you, crit etc?
* Have to understand nature, abilities, gearing and experience of the group for the current instance you are tanking for so you can decide whether to aoe or single target tank? Whether to give mana breaks or chain pull with no downtime?
* Be able to keep an eye on your threat numbers at all times to know if you are losing the threat lead to dps or other tanks and whether to dish out warnings all the while doing your normal tanking activities?
I mean I could go on, but I think you get my point...If you think none of the above applies, then either you do not know the first thing about tanking or you're a great tank and do it ALL intuitively. By the way, Cata will make all of this even more relevant.
As far as anything in WoW is "not hard", in relative terms tanking is definitely the hardest due to the amount of control, adaptability and involvement required to be good at your job. This is one of the reasons even healers eventually try their hand at tanking because both dps and healing eventually lack a meaningful challenge most of the time. Of course, whether they becomes good at it is another matter entirely since as shown above it requires both a large amount of awareness, confidence and ultimately experience.
Also, if you measure difficulty by "rotation" then the only hard spec in the game is the feral cat and possibly shadow priest played well. I hope this is not what you meant and was just a throw-away comment.