People trying to say Drk is just fine really don't know anything about the class. Sure Drk isn't terrible or isn't Warrior 2.0 bad, but they far from what War / Pld is right now. War / Pld tools are far better in just about all aspects.

Below are ability lists so you can compare without needing to do a bunch of research, and come to a better understanding of how all three tanks stack up to each other.

I can't even compare the aspects for this as being remotely even, well lets start with Defensive capabilities? Not going to cover cross class skills with this.

Pld = Shield Oath -1GCD- Reduces damage received by 20%, while lowering damage dealt by 20% and increasing enmity. Also increases chance to hit by 5%.
Shelton -30s- Blocks the next physical attack for 10s. Additional Effect: Partial MP restored upon block (restores 707 MP at lv 60), Better Def and less Cd then our Carve and Spit.
Divine Veil -150s- Upon next HP recovery via healing magic cast by a party member, a protective barrier is cast on all party members within 15y for 30s.
Barrier Effect: Prevents damage up to 10% of PLD's max HP for 30s. Drk / War don't have anything like that.
Clemency -1GCD (MP 2121)- Restores target's HP. Cure Potency: 1200 - Additional Effect: Restores to self 50% of HP restored to target if target is a party member.
Rampart -90s- Reduces damage taken by 20% for 20s.
Convalescence -120s- Increases HP restored by spells or actions by 30%.
Awareness -120s- Nullifies the chance of suffering critical damage for 25s
Sentinel -180s- Reduces damage taken by 40% for 10s.
Bulwark -180s- Increases block rate by 60% for 15s. (Which block is so much better then parry)

War = Defiance -10s- Increases maximum HP by 25%, while lowering damage dealt by 25% and increasing enmity. Increases HP recovery via curing magic by 20% for self. Also increases chance to hit by 5%.
2% Parry per stack (5 total)
Raw Intuition -90s- Parries all attacks taken from the front for 20s. All attacks taken from the flank or rear will result in critical damage.
Equilibrium -60s- Restores own HP when used with Defiance. Cure Potency: 1200. Restores 200 TP when used with Deliverance. (All tanks should have a form of this)
Bloodbath -90s- Converts 25% of the damage dealt by next successful offensive ability into HP - Buff duration: 30 sec.
Foresight -90s- Increases defense by 20% for 20 sec. (nothing special)
Vengeance -120s- Reduces damage taken by 30% and delivers an attack with a potency of 50 every time you suffer physical damage for 15s
Thrill of Battle -120s- Increases maximum HP by 20% and restores the amount increased for 20s.
Inner Beast -1GCD- Delivers an attack with a potency of 300. Ignores the 25% damage penalty inflicted by Defiance. Additional Effect: Absorb 100% of damage dealt as HP Additional Effect: Reduces
damage taken by 20%

Drk = Grit -1GCD (MP Cost 1300+)- Reduces damage received by 20%, while lowering damage dealt by 20% and increasing enmity. Also increases chance to hit by 5%.
Shadowskin -90s- Reduces damage taken by 20%. Duration: 20s
Dark Dance -60s (MP Cost 1768)- Increases parry rate by 30%. Can only be executed while succumbing to the Darkside. Duration: 20s - Dark Arts Effect: Evasion increased by 20%
Dark Mind -60s (Mp Cost 1768)- educes magic vulnerability by 15%. Can only be used when succumbing to the Darkside. Duration: 10s. Dark Arts Effect: Increase magic vulnerability reduction to 30%.
Shadow Wall -180s- Reduces damage taken by 30%. Duration: 10s

Now this is only what each class has base without cross class CDs. Soooooooo

Pld = 8 CDs / Cross = 10 Cds
War = 7 Cds / Cross = 10 Cds
Drk = 4 Cds / Cross = 7 Cds

Pld / War even out quite well but Drk is 3 Cds short. Having a Pld tank stance without a shield for added Mitigation, kinda hinders Drk. Case and point is that Drk just isn't as good nor ever will be as good as Pld / War until those issues are fixed. (Stance and Cds)

This is only the Defensive side of the issue.

Now for dps potential: This is mainly for single target!

Pld = Sword Oath -1GCD- Increases the potency of auto-attacks by 50. (quite powerful)
Goring Blade combo -3GCD- 920 potency / Mp regen (Takes 24sec to achieve this potency)
Royal Authority combo -3GCD- 690 potency / enmity combo
Rage of Halone combo -3GCD- 610 potency / enmity / Utility combo (-10% str on target)
Circle of Scorn -25s- Delivers an attack with a potency of 100 to all nearby enemies. Additional Effect: Damage over time for 15s, Potency: 30 (combined total potency of 250 per target).
Fight or Flight -90s- Increases physical damage dealt by 30% for 30s. (33% up time)
Mercy Stroke -90s oGCD- Delivers an attack with a potency of 200. Can only be executed when target's HP is below 20%.

War = Deliverance -10s- Increases damage dealt by 5%. 2% Crit Rate per stack (5 stacks)
Storm's Eye combo -3GCD- 610 potency / 20% dmg up / 10% slash resistance down / 50% reduced healing (so gets more powerful then what the potency would say)
Storm's Path combo -3GCD- 590 potency (again 20% more dmg / 10% slash resist down so more powerful then potency tells) 50% dmg done heal / -10% dmg from target
Butcher's Block combo -3GCD- 630 potency / enmity combo
Fracture -90s-Delivers an attack with a potency of 100. Additional Effect: Damage over time with potency of 20 for 30s- potency 300
Fell Cleave -9GCD (or with Infuriate every 60s)- Delivers an attack with a potency of 500.
Brutal Swing -20s oGCD- potency 50 / Stuns for 3sec
Berserk -90s- Increases attack power by 50% for 20s. Unable to use weapon skills for 5 seconds after effect fades.
Mercy Stroke -40s oGCD- Delivers an attack with a potency of 200. Can only be executed when target's HP is below 20%
Internal Release -60s- Increases critical hit rate by 20% for 15s.

Drk = Darkside -5s (MP 443)- MP is slowly drained while increasing damage dealt by 15%. MP refreshing statuses have no effect.
Souleater combo -3GCD (MP cost if DA used 1768)- 660 potency normal / 800 potency with DA / MP regen
Delirium combo -3GCD- 680 potency / MP regen / -10% Int on target
Power Slash -3GCD (MP cost if DA used 1768)- 670 potency / enmity combo / boosted enmity with DA
Scourge -1GCD- Delivers an attack with a potency of 100. Additional Effect: Damage over time with a Potency of 40 for 18s (30s with trait) (combined total potency of 500 with trait)
Low Blow -25s oGCD- Delivers an attack with a potency of 100. Additional Effect: Stun Duration: 5s Additional Effect: 30% chance that next parry will reset the recast timer
Salted Earth -45s oGCD- Creates a patch of salted earth, dealing unaspected damage with a potency of 75 to any enemies who enter. Duration: 21s (combined total potency of 525 per target)
Plunge -30s oGCD- Delivers a jumping attack with a potency of 200.
Carve and Spit -60s oGCD- Delivers an attack with a potency of 100. Also restores MP if used while not under the effect of Dark Arts. Dark Arts Potency: 450
Blood Weapon -40s- Increases attack speed by 10% and converts physical damage dealt into MP. Duration: 15s
Mercy Stroke -90s oGCD- Delivers an attack with a potency of 200. Can only be executed when target's HP is below 20%.

Now it may seem like Pld doesn't bring much to the table on the lack of abilites, but there pure potency and stance / buffs are powerful. Making them very consistant, on the other hand War / Drk seem quite similar. Not close really if you get down to it.

War has a very fuild rotation and great buff uptime to further increase there deadly tool kit. Out of the 3 tanks they have highest burst hands down. Probably best OT in game considered by most, even though I think Pld can make them work for that title.

Drk has on paper quite a great dps tool kit (until you release MP is a pain in the ass to manage along with TP), but in practice it is clunky and not effective really. Button bloat is more trouble causing then helpful. DA is clunky and punishes you more then helps. While Drk on paper shows them to be quite powerful and consistent for dps, in reality this isn't the case at all.

I really hope this shows the overall short comings of the Drk class, and people can now better address the issue with knowledge at hand.