I out tanked a lot of Warriors in TBC, and done the same for many in WoTLK, been better geared, at the end of the day it doesn't matter.
As an MT you need to be able to:
1) Learn your role fast
---- If you can't go into a fight having read wowwiki or similar and understand 90% of your role, you take a back seat because the main tank isn't allowed to get it wrong.
2) Analyse fights
---- Your advice / thoughts might be ignored, but your job is to notice stuff that affects you and how to minimise it, again if you can't you don't mt.
3) Lead the raid
---- A lot of MTs are raid leaders, precisely because of 1 and 2, if you aren't comfortable talking to your raid then there is a problem because other people don't watch what the tank is doing the same way you watch the raid
4) Be experienced
---- Helps if you know the fights, the alt tactics etc. The number of groups I have taken through oculus who went "third boss is hard", "omg thats retard proof" is stupidly high, you need to know the fights and know them well.
5) Be geared
---- Gear is the last thing you need, progression guilds don't down Naxx wearing 2/5 T7.10 + best heroic gear + badge gear, they down bosses in what they have that meets the minimum requirements (because getting more takes time). Gear is a big stick people talk about, and frankly it is over rated, important, but if you need 29k to survive the boss and you have 35k all you have is a bigger buffer letting others make more mistakes, and sometimes it doesn't matter. If 28k = 2 hits, 28.1k vs 35k makes no difference because the third hit still kills you without good healing.
6) Prove it.
You can be the best geared tank in the world, and I can still think of you as an idiot (I knew guys in full T6.5 I considered bad tanks, and guys in T4 I considered good tanks), getting to be MT in a guild you aren't raid leader or guild master of is hard work and takes time. Pulling stunts like pulling off the current MT is bad form because it shows you don't do 1/2, and failed at 6. Yes your JoL can pull, but sitting comfortably in 2nd position and turning off Righteous Fury is a much better solution than pulling and possibly dying (a lot of Tank healers watch the tank, not threat, and don't use /assist healing macros - ie: pull can equal dead). Be in the position to save the raid, be competent, be efficient, and at the end of the day be patient, you may never be MT in a guild, a lot of Tanks don't ever leave, and they are often trusted to do the job because they did 1,2,3, and 6.
Edit: I have forced my way into the MT role using numbers before, but only because the MT was willing to listen and understood that for those fights I was simply the better choice to tank mechanically. If you are demanding to tank on RoS pt2 equivalent fights, well good luck, but demanding to tank pick your fights, pick ones where you have a mechanical advantage first and explain it as better for the raid as a whole.
I was RL for a heck of a lot of ZA progression and Bear runs (with the guilds official MT there as well), and in my whole time I MT'd Jan'alai and Halazzi twice because it was mechanically better for me not to do so, I also tanked Akil'zon because I could do more threat, and solo tanked Nalorak because it was fun (well 89% of a fight, and 75%+ of a few others) because people were happy and understood that I was making the right choice to tank or not even if they didn't understand it. Gain that trust, nothing better than having a healer say "pull all 8 next time rather than 4" (2nd pull of Heroic UK

) because they feel that you can, and also that they trust you to do it.
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Armoury Link
