Wanted to call out this out as this isn't something I specifically haven't seen called out as an issue by other DRKs, so I would like to see the opinions of other experienced DRKs.
In my experience as a DRK off-tank I feel a major issue with the class is that Grit MP cost is prohibitively expensive. I get that on paper the 1326 MP cost is designed to be in line with the double 884 cost of switching from Paladin Shield and Sword Oath and back. The problem is that mechanically it is far more taxing on Dark Knights to switch in and out of tank stance compared to Paladin and of course Warrior.
The first problem is that generally switching into Grit is commonly followed with a DRK being in damage mode meaning they had been fueling their attacks with mana. Staying above 1326 mana is critical in a lot of situations were you will need to quickly get into Grit to either take some damage or to quickly pick up and add or tank swap. The difficulty of this is just magnified with the constant MP drain of Darkside and without a decent buffer, you risk wiping the raid by not having enough mana for Grit.
The second problem is when you actually do switch into Grit. Much of the time you either need to move right into the power slash combo meaning you aren't making progress on restoring your massively depleted mana by using Grit. You can of course pop blood price but with a single target attacking you, you usually only get a little over half of the mana grit cost you to begin with in the 15s duration. This of course is the best scenario here. A lot of times you need to just pop on Grit and absorb some damage; orbs in Alex4 for example. It's just a huge overall cost of mana in this situation to try and dance in and out of Grit AND fuel your attacks.
I would really like to see the cost of Grit halved to closer to 600-650 MP at level 60. This would make Grit dancing a lot more viable and bring the switching mechanics in line more with PLD who doesn't have a huge reliance on MP to begin with and WAR which of course became stance switching masters post level 50. Keep in mind that when enabling Grit, it is still on the GCD and breaks combos.