Going to the next level: CDs, Positioning, and Incoming Damage
It is often said that the difference between a good tank and a great one is knowing when to use CDs, and by extension, how to place the raid and when things are going to happen. I've touched on this above, but now I want to talk about all of these points together.
Actually, there isn't much to say. Once you learn a fight, once you know when things will happen in the fight, then you are able to be PROACTIVE rather than reactive. Using CDs just before damage comes in. Positioning the raid immediately after the ground changes color. Knowing what damage is avoidable by moving out of the targeted zone will all go a long way to making the fight easier.
And for healers, damage never taken is the easiest damage to heal. If you know a boss is going to use a 300k attack on you and you have 150k HP, you know you're going to be hurting. With 58% damage reduction from armor, that 300k hit will actually end up hitting for 126k (assuming it can't be dodged/parried or wasn't dodged/parried). 24k health - not much. But if you pop Shield Block, you're guaranteed at least a 30% reduction if you're at 72.4% combined, before armor (at least I think it's before armor). So 300k becomes 210k (30% off the top) and 210k with 58% of it reduced next due to armor is now just 88k that came through your block and armor. That leaves you with 62k health, quite a bit more (more than twice) than if the attack came through without using a CD. That's 38k damage you never took and thus your healers never have to heal.
With this in mind, hopefully it's a little clearer why knowing when damage is coming and moving to avoid avoidable damage is a mark of a great tank. By removing damage taken from the table, you are putting less stress on the healers and increasing the efficiency of the raid.
Okay - what was that unhittable thing you talked about before, and what does 72.4% mean? Unhittable is a misnomer term used by the tanking community to talk about when you have a 100% chance of at least blocking the incoming damage. Basically, if you're unhittable, then every incoming melee attack is either avoided or reduced due to block. It's pretty simple. The math get a little more confusing.
First, the unhittable 'soft cap'. The unhittable soft cap is 72.4%. That means your Dodge + Parry + Block = 72.4% Why 72.4%? Well due to the 3 level difference between a raid boss and you (they are level 88, you're level 85) the boss has a hit table against you that is 100%, but from your vantage point, 3 levels lower, it is 102.4% This means that in order to guaranteed that the boss misses or you dodge/parry/block every attack, you need dodge + Parry + block = 97.4 with the base 5% miss chance of a boss making it an uneven 102.4%.
The 72.4% 'soft' cap is determined because Shield Block gives you another 25% block chance for its duration. So if you have 72.4% dodge + parry + block, plus shield block up for 72.4% + 25 = 97.4% chance to dodge/parry/block every incoming attack while shield block is active. Add in the base 5% chance for the boss to miss you and you have 102.4% - unhittable.
But this is a soft-cap. You need a CD to reach it. You can't hit 102.4 Dodge/Parry/Block/Miss without shield block. You can't even in the best gear available in 4.0.6. I think ~95% is as high as you can get. So you cannot currently hard-cap unhittable, though it's estimated in the next tier you will be able to (so whichever patch includes Firelands).
Does this mean you should aim for 72.4%? My 2¢ is no. Keep going, get dodge + parry + block as high as you can. This decreases the number of attacks that come in at full strength making it easier for healers to keep you up. But understanding what Unhittable means is important, even if the term itself is a misnomer.
So why gear for unhittable (maximizing mastery) instead of avoidance (maximizing dodge and parry)? The reason is healibility. While it is true you will take less total damage if you pump dodge and parry, (100% > 30%/60%) you will also take more full power swings, meaning your healers will need to use more mana intensive heals more often. Smoothing out the damage, even if you take more of it, will allow healers to use more efficient medium heals and save their CDs not for a bad string of full-power normal attacks that got through, but for tank-killing burst abilities.
Essentially smooth damage intake makes healers happy. Having to react to unexpected bursts of damage from a string of full-power attacks makes healers out of mana sad pandas.