I know have read more of the posts GC commented on... my my, warriors and DK's are really screwed it seems. They all feel they can't MT because of less Effective Health or mitigation... well...
mmm... nah nah nah nah nah (sorry, couldn't resist)
My opinion is simple on this matter. The MT of a guild is not the one with the highest Health/mitigation/whatever. He is the one the whole raid trusts. I have been the main tank for XII for over a year, in TBC, where tankadins had lower everything except AoE-threat. So why did XII raided with a Tankadin? Warriors where still better suited for almost any boss, so in theory making them easier. It's called trust.
In all those raids we did, I may have made five mistakes...and the reaction of the raid was; 'yeesh, he is human after all'. I was the MT, becasue I knew the fights, I let other tanks do some bosses, but every time I said 'I want to tank this', no one complained, even if the boss was a nightmare for tankadins (Maiden anyone??). The raid knew I would do it, they may had to heal me a little more, but never was that the reason for a wipe.
The Main Tank, does not need the best possible mitigation/health/threat, he has to inspire people, be trustworthy, and be very confident about what he can do. Main Tanking is a mind-set, always be the best at what you do, but also you need to watch out for over-coinfidence, and you need to watch out to not become a dickhead. Remember, every tank wants to MT a boss, so be carefull to let them do that. That way you will generate a Tank-Team. And if all the Tanks in a guild are good friends...poor pixel-bosses.