For me, it's the balancing of the needs of tanking - some DPS output, survivability and mitigation, and hate control - balanced with the core identities of the three tanks.

- PLD was the tank that was meant to feel more like a healer, with good defense to the rest of the party, good self sustain, and the ability to heal others (identity KEPT and IMPROVED in 4.0)
- WAR was the tank that was meant to feel more like a melee DPS, with better damage output, higher HP, but a slightly weaker self defense and fewer party buffs in exchange (identity lost in 4.0)
- DRK was the tank that was meant to feel more like a caster DPS, with strong AOE attacks, cast based attacks, good magic damage mitigation, but little to no party utility (identity lost in 4.0)

WARs are on their way to getting their identity back. DRK is still lost in the woods. We were the king of AOE threat in 3.x content and we had some of the best low level damage mitigation available. Now we have the strongest short term damage reduction, which is great, but we're defenseless too much of the rest of the time.

DRK may still be the strongest AOE threat generator, but the job doesn't have much of a place in endgame raiding against single targets with that as its one major asset.