Karazhan/heroic tanking stats.

Stamina/HP: 12000hp
This is an acceptable start for raiding. If you're just a little bit below you can still manage karazhan, Just keep commanding shout up. With commanding shout I have just a little above 13k, the more health you have the better though, for it helps out healers. Again, you can survive karazhan just a couple 100 HP below 12000.

Defense: 490
Having 490 defense and above is the only way to be uncrittable for karazhan/heroics. If you do not have this much it is advisable to get the helmet glyph (revered- CoT) and a few enchants/gems before entering kara. Your healer will be tempted to quit if you get crit for a lot of your health. If you don't have 490 defense it is probably true you don't have 12k health, either. This means you can't take much, and if you die the other tank will struggle to pick up the threat on the mob that killed you.

Dodge/Parry/Block: 15%
To be 'Uncrushable' make sure your dodge block and parry are all at 15% and up. Always keep shield block as well. This will make you take less damage as well as keep you from being 2 shot.

Armor: 13000
This is a decent starting amount of armor for kara to help mitigate as much damage as possible. 12000 is acceptable and can be dealt with, just keep your mittigation skills up as much as possible, this will be easier to deal with in an off-tank role. As with all tank stats, more is better.

Credit to:
http://forums.worldofwarcraft.com/thread.html?topicId=3421023546&sid=1