This is purely speculation, but I don't believe shadowknights will be dishing out extremely high damage. From the way they have been presenting the class as, "a warrior who inspires fear in his enemies," it seems the class will rely more on debuffing and other fear abilities.
I'm expecting alot of abilities that 'cripple' an opponent with terror. I think the Dread Knight might not be able to hold aggro as strongly as a warrior, but when the target bounces around, that he will have sufficiently weakened the mob through fear and other deteriorating abilities that the damage to the other members of the group will be manageable. And like QuinnDexter said, "Very high attack/debuff ability for a short time," sounds promising.
From the reports by the designers (Though they are extremely vague) I'm expecting many new views on the concept of tanking and playing any class in general.