The problems at least for me in the current state of the job are these:

-> When mitigating you have too many buttons to press for little gain ( ex: Dark Mind , Oblation ). While Dark Mind is good against magic damage and it has a low cooldown of 60 seconds, Oblation adds only a 10% while sharing the same cooldown. It makes absolutely no sense for both of these abilities to remain on a job that is already double weaving during its burst for roughly 8 GCDs straight. If I were to fix this I'd make Oblation grant 20% damage reduction and a regeneration effect for 150 potency for 12 seconds, with its duration also increased by 2 seconds due to the way healing ticks work. The heal over time is kind of necessary because the job has no healing in its kit and sometimes you'd wish you could heal yourself a little bit to not die, every other tank can do this. Dark Mind should be removed as it has no use against attacks that aren't magical, and weaving it along with Oblation and TBN is a pain. Every other tank does just fine against most big hits if they just use their 30% and their personal mitigation ( PLD , GNB ) , while Warrior can do the later but with Thrill, but it has nowhere near the amount of oGCDs DRK has.

->Dark Missionary and Heart of light are the same. Make them slightly different, Dark Missionary could heal over time for 100 potency, while Heart of light could heal whenever you deal damage.

->Abyssal Drain and Carve and Spit should be lowered to 30 seconds cooldown for a couple of reasons. The job is very boring when its not bursting ( every 1 min, and every 2 mins ), so having to use either one of them every 30 secs would make it more interesting. In dungeons this would mean you'd get to use abyssal drain more often per pack ( twice ). The damage should be cut in half to make up for it, however please buff Abyssal Drain's heal potency to 250, or make its area of effect larger ( 5 y -> 8 y ) so that it hits every mob pack easier.