Not much really. DRK has no utility. WAR can burst damage and heal. WAR takes hits roughly the same as DRK but with the Defiance hp boost it is sturdier. DRK has TP/MP issues as well. PLD is the king of tanks due to defense/cooldowns/utility like cover, sheltron, and clemency? the heal, and still can do decent dps.
Right now there's not and high end game content until Alex and then it'll become apparent how much DRK needs love.

This is coming from a 60 DRK who has seen all content to now. When you're in a grp with a PLD or WAR you can see the difference.