I feel like DRK is very strong defensively. They have one of the best (if not the best) mitigation kit out of all the other tanks. It's between DRK and GNB. The other tanks needed upgrades to their short cooldown mitigation tools to be on par with DRK. I won't go into details on high-end content since you're looking at DRK from a casual perspective. Despite the other tanks' upgrades, TBN is still currently mitigating more damage compared to the other tanks' upgraded cooldowns. One of the hardest single target tank busters right now hit for about 47,000+ damage. With all tanks being equally geared, TBN is soaking 8% more damage than WAR's Bloodwhetting (20% + approximately 8% shield), 13% more than GNB's Heart of Corundum (15% + 15% but due to scaling it's approximately 28%), and 7% more than PLD's Holy Sheltron (15% mitigation + 20% block = 35%). That's without even having to pair TBN up with Oblation. The other tank's kit does include self-sustain in some shape or form though which is something DRK doesn't have with TBN. Don't forget that TBN is on a 15s cooldown as opposed to 25s on Bloodwhetting and Heart of Corundum as well as 22-23s on Holy Sheltron (assuming the PLD has something to auto the entire time).

Technically, WAR and PLD lack more defensives compared to GNB and DRK having 1 or 2 additional options. Outside of the role actions, WAR has Vengeance, Bloodwhetting, and Shake it Off and PLD has Sentinel, Holy Sheltron, and Passage of Arms. It's just that WAR self-sustain is just so insane (in wall-to-wall dungeon pulls) that people don't realize WARs are taking huge spike damage. If they run out of mitigation while waiting for Bloodwhetting to come off of cooldown, they are in a very bad situation. Same for PLD. Since PLDs don't have a powerful self-sustain kit like WAR, you can see and feel how much they are struggling to keep themselves up due to no mitigation. Raid scenarios are by no means "low damage scenarios". Boss auto attacks in Savage hurt...a lot. Even the autos on the third floor of Pandaemonium hurts quite a bit right now and that's only normal mode.

DRK needs quality of life changes though, such as an improvement or change to Living Dead. Normally I wouldn't say this because healers can typically top off the DRK (just requires more work), but, with SGE not having an "Oh shit" heal button, AST getting a nerf to Essential Dignity, and SCH's "Excog", not having much of an impact in Living Dead situations, Living Dead needs to be changed or at least have an additional effect where healing done to the DRK is increased or it takes no damage to give healers some breathing room to heal the DRK. Only WHM's Benediction is capable of topping the DRK off in a short time (instantly in this case). Blood Weapon needs a fix. Perhaps have it on a stack system.