A long while back, our guild had some growing pains to say the least. During the lull between BC and WOTLK there was a serious lack of tanks. Our main tanks would log on for raids then promptly be missing for the rest of the week. This led to a huge rash of players building up tank alts who eventually became mains.
All these new mains then led to a bigger problem in the form of drama. They all wanted to be rotated in or out in out considered as a main tank. So while poking through our old forum posts, I found a write up on the state of the back up tank.
I will be honest with you, being a tank has its share of politics. Its not the end of the world, and most every guild I have been in has its share of it. Guilds have their Main Tank(s). These guys are the ones who the guild turns to for direction, movement, and solid beefiness. These guys respond to the situations at hand, and lead the raid into battle accordingly. A lot of things ride on their shoulders, and because of that, a lot of trust is put with them and from them.
Breaking in to be a Main Tank does not happen actively. Over time, things just happen within a guild and they see who they trust. If you are coming into a guild or raid to be a "Main Tank" walk away now because that, in my opinion, should not be a goal. Doing the job right and well is priority.
More realistically, a guild already has their main tanks. What you then are is considered Off tank which could mean second or third string back up. Imagine being the back up QB for a pro football team? The "real quarterback" gets all the glory and face time, and you sit on the sidelines tossing a ball back and forth to keep warm.
Now, a back up tank is an important job as well. The guild has been progressing thus far with their main tanks, but something happens and they aren't there one night. It is the time for you to step in and fill their shoes. This is the crucial moment for you. Not only are you expected (unfairly, probably) to perform just like the main tanks, but you have to do it instantly and usually with little warning. The guild can not hold your hand and they are putting a lot of faith in you to see the raid through. They don't care that you have as many purples as the main tanks (though, mind you if your stats and mitigation are less than superior, be honest about it and the raid will thank you for it). What they do care about it is "Can we trust this guy?"
Yes, the overall theme. Trust. So how do you gain trust?
There are various ways and folks to gain trust with. The healers have to be confident in you to not kill them. It is a stressful job enough making sure everyone stays alive, and the more you can help them achieve that the better. This means knowing the situations at all times, being able to adjust and flow, and communicate with others when shit hits the fan. This might mean mob positioning, keeping a high threat per second ahead of the dps, and/or watching when adds run off and quickly gaining them back. Sometimes, the tank has to eat some damage in order for the whole raid to not take Dragon breath in the face.
The other tanks have to trust you too. After a while, the main tanks have an understanding with each other. They know each others weaknesses and strengths, and know that if something goes wrong the other tank has their back. Now, a member of their little two man team is gone and some new guy is there. You need to keep communication with the other tank and be willing and open to constructive comments made to you. Tank egos are fragile yes, but if you want to come back the next time, set that ego aside and say "yes". In addition, the other tank doesn't know you, so please speak up if there's an ability or mechanic you are sure to be suited for. The main tank may already understand what you are presenting, but if you present yourself respectfully and cautiously, it will go a long way. (Forcing your side may not work)
Raid leaders are also the guys who you need their trust from. They have so many names and people they are working with, and next to the healers, they have to fill the tank role first. Be patient with them. If you keep screaming "Put me in coach", without bending a little their direction, you might get passed over for someone else. Do not give them drama. Do not give them emo QQ over shit. Be as intelligent and thoughtful as possible. Barrage of tells to the raid leaders will quickly get you ignored. A simple statement of availability and interest seems to go better like "Hello. I know you are busy. When you have you have a second, Id like to bring up something". When you get your reply back something like "I know there are other tanks online. I am ready to go and understand the fights. If you need me, I will be around."
If you have worked your way up with building trust with raiders and other leaders, the time your name comes up it will be looked at in a more favorable light. Still keep in mind that most fights need 4 tanks at the very, very most, and usually just need two or three. How you perform and behave in the small windows of time you get with the guild will go to keeping you in the rotation.