I see a lot of posts complaining about chain pulling tanks. Plenty of replies stating how they never slow down for OOM's. As an off tank in my, I need to run a quick dungeon for my 2 frost badges and move on to either a raid or get to rl, my goal as a tank is to move through a dungeon as quickly as possible. If the healer is at 35% or higher in mana there is nothing in a trash pull that will wipe a heroic dungeon. You don't need 100% mana to heal someone, you do need some intelligence to know how to play your class.
As a tank I am in no way concerned if the dps in my group needs mana. As a warlock or mage or priest if you need mana, sit immediately after the final mob of this pull dies and drink up. If you're a druid and even consider complaining about mana reroll. But don't ask me to slow down on trash.
Please stop complaining about a tank chain pulling. When 30% of the time you have to sit through a heroic where the tank marks every mob, waits to ensure the hunter's pet is happy, checks thottbot for which mob he should focus his aggro on, does a ready check or any of the other things that crappy tanks do.
Granted there are times that a tank has no clue what he is doing and pulls a group and a pat is making it's way around and he doesn't see it because he doesn't know what he's doing. That's just a crappy tank, hope next time you get a different one. Blizzard has provided a wonderful tool where if you don't like someone you grouped with you never have to group with them again. I will never block a tank though because doing so lowers my queue time when I roll dps and I know that if the tank fails I can switch specs and take over the roll.
So in conclusion stop your bitching and enjoy the 20 min heroic.