Skills that only work on a specific type of damage.
The "core" mitigation skillset (Rampart, Sentinel, Shadow Skin, Inner Beast, etc...) is mostly balanced between the three tanks.
But if you put a PLD in a very heavy magical fight, like A1, it suddenly loses a lot of skills, since the shield is useless.
DRK, being the magical tank, really shines with that type of fght, mainly thanks to the powerful and frequent Dark Mind. But, if you put it in a physical fight...well, it still holds fairly well if it can properly time Dark Dance and Reprisal.
WAR has around the same issue as PLD, even though it only loses one skill (Raw Intuition) during magical fights. Yet, being able to use Inner Beast very frequently and being able to heal itself pretty well makes it a strong tank in every situation.
That's why some people are suggesting PLD's shield should be used to block magic with the proper cooldown. And since Sheltron was shown to block a Fireball in the HW benchmark, it makes it a good candidate.
