I've heard plenty of people who've tried to tank, complain about how difficult it is. Actually I don't think I've ever heard someone say "wow, this was fun!" the first time they do it. One could really wonder why people keep at it at all. It's difficult not only skillwise, but because you ultimately are fighting not only the mobs, but against your own group as well.
In addition to the social issue, there are real technical challenges. For example:
- Tanks have to understand the dungeon mechanics so that they can mark targets and choose pulls. In theory someone else can do it, and I've even seen a few groups where it went that way, but most of the time everyone stands around waiting for the tank to decide what to do. As a result, tanks have to understand all the patrolling groups and all of the enemy mob abilities, and what mobs are linked to each other. The other players just come along for the ride.
- During a fight, tanks have to be aware of more things than the other players. In addition to things all player have to watch, like which abilities are on cooldown and where the fires are that need avoiding, they have to know where all the mobs are, whether a CC has broken, whether a new add has wandered into the fight, whether the healer has aggro, how threat is looking on all the mobs, and what level the tank's own health bar is at.
- Tanks take care of more issues than other players. In addition to maintaining their threat rotation, they have to taunt mobs, trigger defensive cooldowns, keep an eye on ranged mobs, play line-of-sight games, switch targets to mobs where threat is decaying, and position and reposition mobs.
- Tank death is less forgiving than other players dying. If a healer dies halfway through, many encounters can be survived by a tank blowing all their cooldowns. If a dps dies, it really doesn't matter. If the tank dies, though, the monster had better be very close to dead or it's a wipe.
I like tanking, but it was a lot to think about the first few times I tried. For anyone wanting to try it, be sure to try easier content first. One way to do that is to start a new character and level by tanking dungeons. Blizzard has balanced low-level dungeons to be rather forgiving, probably because they assume that the players in them might be playing their first toon. Another way to make a dungeon easy-mode is to switch roles with a max-level toon, but to first gear them up before doing so. Use excess justice points to get some tank gear, and then jump into some normal, non-random dungeons.