Most people who claim Drk to be OP are those that are still in 'awe' of it just being the new tank class, and are very likely still doing early level 50 stuff, if not lower.
Any tank can tank that stuff fine, without much difference really.
Yes, CD's should be used regularly as a Drk, especially Shadowskin and Dark Mind due to the low cd's.
The problem comes in when you look at the layers of defense each tank has. As dank1 said, it's about the balance, and as is, Drk is at the very bottom due to this.
Warriors have the benefit of extra HP, extra healing, and the parry chance. They're a reactive tank when it comes to Inner Beast, but a good Warrior will ALWAYS have Inner Beast when they need it. Essentially giving Warriors exactly what Grit gives a Drk (Excluding the acc), for 6 seconds, when they need it the most, ONTOP of their Defiance. Which is all that matters.
Paladins have an entire shield mechanic which involves blocking crazy amounts of damage. That and their tanking kit is really the best as far as safety net tanking goes. I'd even go as far to say that even in an OT position, Paladin outshines a Drk easily, due to their new toys.
Drk's simply have a 20% mitigation stance, same as Paladins, that also increases our chance to hit by 5%. But we do not carry a shield, thus we cannot block, and we do not have the Parrying capability a Warrior has, nor anything equivalent to the reactiveness with their Inner Beast. It's like we're missing what we need to make us shine compared to the other two.
It wont be that noticeable until you're at 60, with other tanks. Tank busters, and high quick incoming damage can really screw a Drk even with their buffs. A Pld can block and pop cd's, a War can Inner Beast with cd's for extra layer of protection. As a Drk, we pop 1 CD and hope to god we don't need to pop the other, because chances are we're going to need it for something else. This also leads to us sitting on our CD's waiting until we need them the most, even if we're taking boat loads of damage at the time. For example, during the 2 snake-thing adds in Bismarck EX, I find myself sitting on CD's, and trying to only use 1 at a time between each tank shift, because the moment we switch, I know I'm about to get nailed by a dry fin, and wet fin.
In Ravana EX, well there isn't exactly a tank buster in there really, but eating a cleave split with the other tank, unmitigated, can still hit up to 9k easily. With the amount of outgoing damage there is in that fight in quick bursts, it can really suck. Not quite as bad as it'd be if it had a tank buster other than a cleave you split with the other tank though. A Paladin however, and possibly even a Warrior, can solo tank this fight. As for a Drk, well I'm not entirely convinced they could. I doubt it seeing as you have nothing but 2 small mitigation cd's, one of which has a stupid high cooldown for what it does. Perhaps in much, much better gear, but as is, that cleave left unmitigated can smack you for 14k easily. Add that in with the rest of the outgoing damage and what the rest of your party needs to watch out for, and it just becomes overwhelming.
You must have never met a geared Monk before. Or a Dragoon.
Hell I watched a Dragoon tank almost 90% of The Aery, up to the last boss.