Dark Knight will probably have an offensive stance and deal damage comparable to Warrior, if not more damage. Each class has it's own strengths, perhaps DRK will excel at being on Main Tank for shorter periods of time so the heavier tanks can tank longer, so the DRK can dps more.

We don't know.

But I am REALLY glad it's a tank. DRK being a tank mean less people will confidently play it, and since I've got WAR and PLD, I'm so ready to play my favorite job.