Simplified, the formula becomes: DR% = Armor / (Armor + (467.5 * AttackerLevel - 22167.5))
Consider the following table which can be derived from the above formula for a lvl 70 tank taking 1000 DPS of "raw" damage:
Stat Base AC1 AC2 AC3 AC4 AC5 AC6
Armor 0 5k 10k 15k 20k 25k 30k
DPS taken 1000 666 500 410 350 300 260
DPS reduced by the last 5k armor 0 333 166 90 60 50 40
Relative DPS reduction by the last 5k armor 0 33% 25% 18% 15% 14% 13%
As can be seen, the effectiveness of adding another 5k of armor is getting lower, there is a "diminishing returns" effect with respect to the DPS reduction. It isn't as pronounced as it may seem looking at the absolute numbers, though. The last line shows that the relative value of 5k armor drops from 33% to 13%, meaning that at the start, one point of AC will be about three times as effective in terms of DPS reduction as near the end. Thus, armor exhibits diminishing returns with respect to the total amount of healing needed to keep a tank alive.
The following is the same formula for a lvl 80 tank against a level 83 mob
Stat Base AC1 AC2 AC3 AC4 AC5 AC6 AC7 AC8 AC9 AC10
Armor 0 5k 10k 15k 20k 25k 30k 35k 40k 45k 50k
DPS taken 1000 769 625 526 454 400 357 322 294 270 250
DPS reduced by the last 5k armor 0 231 144 99 72 55 43 35 28 24 20
Relative DPS reduction by the last 5k armor 0 23% 19% 16% 14% 12% 11% 10% 9% 8% 7.5%
Armor vs. Effective Time to Live for Level 70 Characters
However, in terms of absolute time to live with respect to melee attacks, armor has no diminishing return effect. Given a constant melee DPS amount, each additional point of armor (whether it be from 0 to 1 or from 30000 to 30001) will increase the tank's time to live by the same effective amount. 1k additional armor increases time to live by approximately 9.47% (6.01% at level 80??), as shown by the graph. The formula for calculating time to live with respect to melee attacks is:
Effective time to live = 1 / (1 - DR%) *
Base time to live
Where DR% is calculated according to the above formula. In this way, armor can be thought of as increasing the effective health of the tank with respect to melee attacks. Since DR caps at 75%, effective time to live caps at 400% of the tanks base time to live (time to live if the tank had no armor).
Prior to hitting the cap, armor is certainly a very desirable stat for tanks, but difficult to find. Most items of similar quality and rarity have similar armor values, and extra armor can be found only very rarely.