I gave up on tanking.
1. You have to know the fights exactly without ever doing them before (videos only go so far)
2. You have to have the gear without actually having the gear from downing the bosses a bunch of time before
3. You have to contend with DPS/healer that expect you to have #1 and #2
I have tanked on various toons on/off since Vanilla, including MT and raid leading 40 mans all the way to some MoP LFR.
It's just not worth the headache.
I've spent far, far too many nights in raid smacking a boss in the crotch while the rest of the raid actually gets to see the fight and participate in the fun mechanics.