My info is anecdotal, so take it with a grain of salt.
Palys generally have the lowest HP for a tank – all things being equal.
Druids have the most HP for a tank – all things being equal.
DK and Warriors are somewhere in between – all things being equal.
To compensate for the lowest HP, Paly tanks are given the most effective health. (Basically – my understanding of “Effective health” means that the Paly might have less HP, but be able to take more damage due to being able to avoid and better mitigate incoming damage… EXAMPLE: Warrior tank with 30k HP will be able to tank through, let’s say, 35k damage. 30k of Health eaten and 5k that he will dodge, parry, block, etc… A Paly will come to the party with only 29k, but be able to tank through 36k of damage. This is due to their ability to be Def Cap’d (Generalization for being Crit Immune and a standard for ALL tanks), but also because Paly tanks can be Block Cap’d at all times.
I don’t know the statistics, but if Paly tanking is done correctly – you can effectively be at something like 102% chance to mitigate damage (through something other than just plate armor mitigation – aka the 5 D’s of dodgeball: Dodge, Duck, Dip, Dive and Dodge.)
Someone like Losie should further explain this, but I think my generalizations are pretty accurate.
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