I agree, it should be the most interesting mechanically. To me, dark knights have sacrificed their own hp/mana/defense in exchange for power, using dark magics to cripple or drain resources from an enemy. Currently we sorta do that with syphon strike and souleater, and both of these are either enhanced by or only available in the defensive stance, grit. I would like to seem them play up our identity more than button mashing "dark arts" and using three absorbtion abilities. We'll see what comes in coming weeks, but I think dark knights are in worse shape than people think, I think when people begin to optimize they will find dark knight is worse off than warrior atm.