Okay, so you've thought about it, and you think you might want to join the Tanking Core. Be you Druid, Death Knight, Paladin, Warrior, or even Shaman (I have seen a few, even today), it is important to make sure first that you can handle the job, mentally.
Now, I don't refer to your IQ when I say this, but rather, your personality. Unlike DPS, or even Healing, there is a certain mentality a player should have in order to be a successful tank. First and foremost a tank, even an off tank, must be willing to lead, both in leading the pace of the group, and in keeping moral up when things are going less than stellar. To survive as a tank, you must be thick skinned.
You have to have a willingness to accept good criticism, and shrug off bad criticism. Mostly, you'll be receiving the bad, just shrug it off. I can't tell you how many times some random dps just decides to run off and aggro the rest of the room, then starts screaming at you to hold aggro. Or they'll think that because you've attacked target A, they can go ahead and go full blast on target B, which is right next to A, but hasn't actually received any damage yet. Or, once you're geared high enough, you begin to pull the room, and they start in on the pack you've already run past, meanwhile the healer has begun spamming heals to try and get a head start on what's sure to be a large cluster****, so now you have to use your taunts on the healer, and pray that the heals are enough to keep DPS up while you re-establish aggro, because you just know one of the DPS are going to have a cow if they should happen to die. . . you get the idea. Understand that for the most part, their mentality is: "If something bad happens, it's the tank's fault."
Helpful tip for those situations, by the way, is to communicate from the very beginning, a lot of times this helps prevent the issue from happening in the first place. A simple, "Hey, I'm going to do X, please don't start in with damage and heals until you see me do Y." (For me, this is usually, "I'm going to pull a lot of stuff, please hold off on damage and heals until you see me stop running around and drop another consecrate.") Which leads to the next key trait a good tank needs.
A good Tank must be able to communicate and ask questions, as well as listen. Even when fighting raid bosses that I could tank blindfolded, when joining a new group for an encounter, it's vital that I make sure I understand how the group normally does it. In this situation, I'm a guest, so it's important that I learn their way, remember that as long as it gets the job done, its the right way to do it. Communication during a fight is also key, particularly if its a fight where you and another tank are trading aggro. Even in a group that I've run with repeatedly, it's alway helpful to call out who is about to take aggro, so that the healers know who to focus on, or when they need to increase their heals per second.
More than the DPS, a tank must have a willingness to learn the fight before hand. Read up on fights you don't know, and actually take notes on what specifically is the tanks job. Take notes on that tank spot video, don't just sit there and watch it. You'll retain what you need to know better if you do. Take notes on the role of dps and healers too, as this will give you better raid awareness once you're actually in the encounter.
Just as in driving, you can't focus on just one spot in the road and expect things to go well. You must be able to check the entire screen. You've got to watch the health of your team, watch your health, watch for important debuffs, make sure no one is catching up to you in threat that shouldn't be (if you're the off tank, make sure you aren't about to rip aggro off your main tank, while still staying above dps) watch for patrols and how close you are to other mobs, and watch your target, all at the same time. Never watch one thing to the exclusion of everything else.
You have to understand and accept that for the most part, tanking is a thankless job. When you do your job right, much of the time no one notices. When for whatever reason you mess up, you can expect everyone to notice. That's just the way it is.
A successful tank must have confidence. You have to be willing and able to exude confidence during encounters, a timid tank is a dead tank. Most tanks I've partnered with have been some of the most arrogant SOBs I've met, and I'm one of 'em too. Pull half the room and the boss too? Absolutely! Remember, if you can pull it off, then what you're expressing is confidence, not arrogance. Now, at the same time, you have to be able to balance this with knowing the limits of the rest of your party. If you start doing stuff to the point where the healer can't keep up with you, you're going to die, and your going to bring everyone else with you, which isn't the sign of a good tank.
So, what's great about being the tank? Everything in the encounter revolves around you. From the pace you set, to where you tank the boss or whatever add your on, it's all made possible because of you. It's the knowledge that even if you aren't listed as the raid leader, for all intents and purposes, you are the raid leader. It's having the knowledge that, even if no one else acknowledges your contribution, that successful run was made possible by you. (okay really it's a group effort, but the tanks know what I mean.)