Defining minimum health is an important concept in end game raiding and groups. For most instances, a minimum of 10,000 to 11,000 unbuffed is needed. This will at least let you take a few blows from a boss before dying, hopefully enough blows that the healer will be able to get off a few heals on you. For more entry level raiding environments, it's necessary to have unbuffed health between 11,000 and 12,000. If you've got 11,500 hit points going into Kara, you'll probably be able to reach 13,000 health fully raid buffed. This will let you survive a good portion of Kara, which means you can get more gear, which begets more health in the long run.
When heading past Kara, the more stamina the better. A main tank will need around 12,500 to 13,500 unbuffed to get by in the first half of the tier 5 instances. By the time a tank is hitting Vashj and Kael'Thas, you're going to need about 14,500 to 15,000 unbuffed health. In tier six the stamina requirements go up even further to about 16,000 to 17,500 unbuffed for a main tank.
It's at that point where the question becomes how much stamina is enough? When you stop having to maximize stamina, you can begin to focus on other stats, like dodge, parry, strength, AP, haste, etc... Small amounts of these other statistics in tanking situations can vastly improve the performance of the tank and increase the success of the raid.
At about 17,500 stamina unbuffed, which is what most main tanks go to Illidan with (although I'm sure there will be those that disagree with me here), you can hit a raid buffed health of around 22,000 to 23,000, depending on group composition, talents, and consumable usage. This is surely enough for most tanks, isn't it? And it is definitely a far cry from the eleven thousand unbuffed that most tanks start off with when turning seventy.
In my opinion, once you hit about 17,500 health you can start gemming and enchanting for different aspects of your tanking class. At this point, if healers can't keep you up with 23,000 raid health, they probably can't keep you up with more. What do you think? Is there a maximum amount of stamina you can have?