We already see the literal hundreds of threads asking for X and Y to be changed and worrying about Y and X not being able to do one thing whilst another does it a lot better. This kind of game design goes back to the flaws of DPS doing more damage against certain creatures, like Black Mage's fire doing more damage to ice creatures. This type of thing was removed for a reason yet..
Why are Warriors good at mitigating everything?
Why are Dark Knights better at Magic?
Why are Paladins better at Physical?
This, is bad! Not only is this a balancing Nightmare but it discourages people from playing what they want. All the tanks should be unique and special and different from each other, surviving in different ways rather then being worse in the same situation.
How can we fix this?
Paladin need a lot of changes, they should still be the best at mitigation physical but I believe the shield should also mitigate magical damage. The lore proves it can when you know who made the ultimate sacrifice. Shields not being able to block magic is a VERY silly concept and honestly this change would be big enough to make the class viable in most of the content. If Shields blocked magic and all its blocking moves also helped they would be very nice in current content. Also giving them more ways to deal damage especially in AOE would be a good change as well.
Warrior doesn't need any changes because its honestly one of the best designed classes in the game. People say its overpowered but its really not. Its well designed and that is that. The only skill I would change is fracture and making their stance actually work for "All healing" even their own.
Dark Knight's should not be so magical focused. In fact that part of them needs nerfed to have better all-around defense. Honestly however they don't have anything unique to them if you remove that part of them, so how can we fix them up to be unique? Simple: Make them a Blood Tank that heals through gathering Hatred and Blood Shields and Lifetaps. Remove the Magical defenses and make them like a Reaver. Remove the RNG mechanics as well. Dark Knight's are way too much like Paladins and they need their own identity. Dark Knight's should also be doing a bit more damage, and their defenses should not be changed with another GCD.
Dark Arts problem: A lot of a Dark Knight's defenses are only good if you use them with "Dark Arts". This is an issue because lets say you see something you want to block, you use your ability but WAIT... you have to Dark Arts it first, so you use Dark arts and SMACK, already been hit with it. That is honestly bad game design and I would rather have another mechanic for the class. That mechanic is not bad for a DPS class, but its NOT good for a tanking class.
The reason i'm honestly bringing this up, is because no other class in the game has to deal with this issue. DPS no longer does more damage against certain monsters. There are no monsters who take more damage from certain moves anymore, and this was a good change. However TANKS are still like this? Why? We can already see its causing problems.