Intro: I tank since 2.0 beta 3. I've tanked in other games as well. My tanking experience in this game is EVERYTHING, including the Savage turns of SCOB. I've tanked with Paladin, Warrior and now Dark Knight, where I was able to MT Ravana X with said class no problem. And even as the seasoned tank I am, I find it super stressing sometimes, because I want to push the class to the limit, but that's my own fault. On my end and my opinion about tanking:

-It requires you to understand how damage works. How much your receive from where and how. This way you know how to manage your cds or swap your stances.

-It requires you to know your limits. How many monsters you pull, what CD you can use, what kind of monsters are a bigger threat (You mark the ones you want the DPS to burn down first). My goal is to let the healer DPS, I don't want him to spam heals on me like a madman. If that healer won't DPS, it will help him to save his MP.

-It requires you to understand how aggro works AND how to track it. This doesn't require any explanation. You don't want all the trash monsters or bosses scattered around, killing the DPS/healers.

-It requires you to know how the other classes work. Don't mess with the boss/trash position unless you need to dodge an AoE. This will mess up with some DPS positionals. Also being careful to not accidentally MPK everybody with a bad position.

-It requires you to have a leading attitude. You're the person running ahead of other 3/7 guys. You're the one controlling the run and how you pull. I never listen to the DPS when the ask me to pull more, but I'll always listen the healer if he/she asks me to.

-It requires you to have quite an understanding of all the encounters you face. If you die by failing at a mechanic, it will probably end up in a wipe. While DPS dying is not an issue unless it's a DPS check, the tank dying will guarantee the group to die. And so the same importance remains if you're a healer.

-It requires yout have a very thick skin. While some playes will be glad to finally have a tank to make their queue pop, others will stare at you with the eyes of judgement. You're the most visible party member, as you are the one fightning the monster and running ahead of everybody. Every mistake you make, will be truly evident. Get ready to receive a shit ton of blame, get criticized and people complaining about tanks being dicks or stuff like that in general. The healer also shares part of this. This rises a bigger issue which was mentioned in this thread. People expect tanks to be experienced already. And yes, it's really REALLY bad when a tank doesn't make his/her homework, as you will wipe more times than usual compared to a DPS who doesn't get it. Sadly, some tanks develop a "dick-ish attitude" to fight back that kind of players. However, sometimes it rises their ego.

-It requires you to proper gear your class. Whereas the DPS can have "ok gear", the tank needs to be updated all the time. If you're undergeared, you might have aggro issues and probably lack survivability as you take more damage than usual.

So, basically, it's like Delorean pointed out. It requires you to have actual responsability. Some people just don't want that as they see this "resposability" as something that might take the fun away. It's a videogame after all. Some people like it, like me or you or the others who posted here. Is is stressful? yah, it is. It's really thankless at the same time. People who tank do that because they actually enjoy it. It's a challenge, we take it.

Sorry for the long post.