The complete answer to this question is pretty long and involves a concept known as "effective health." Look into Tankspot.com (as has already been suggested) to get the run-down.
Basically, the damage you avoid (dodge and parry) and the damage you mitigate(armor and block) and your hps need to provide you with enough survivability to get through the zone. Now that can happen many different ways. You can have really high armor and hps but low avoidance (which happens to a lot of tanks in mostly pvp gear) or can have really high mitigation and block, but low armor and hps (which happens with tanks in instance gained gear) or you can be sort of average all around and in all three cases you can do fine in a zone. What matters is that the whole package adds up to enough effective health for the zone you're going into.
Now it's a bit more complicated than that as each encounter has different needs. Some really are pure avoidance fights, some are pure migigation fights, some are all about the hps. And you'll have to learn which are which and develop gear sets that you can switch out for those fights. But that happens as you go up, not out of the gate.
The one real minimum that is a minimum is defense. 490 is the base, the point at which you aren't going to drop from a critical strike. If you are below 490 defense figure out how to get there, it will make a world of difference in your survivability, regardless of how your other stats are laid out.
Edited, Jun 10th 2008 10:54am by kingpatzer