From continued testing between my FC mates on all three tank classes, it seems the main limiting factor to DRK being squishy and hated in dungeons is the lack of a "gimmick" on their tanking stance.

Example being WAR Defiance have a 20% bonus healing attached to it. PLD, while not having a gimmick on their stance, have the added armor and stats of a shield.

DRK has near the same tanking style as a WAR (Big weapon, no shield with average mitigation (less mitigation actually, if you count inner beast being always up and drk reprisal having an actual cooldown)) but has nothing attached to it's tanking stance to help offset the lack of a shield, as in a PLDs case.

My suggestion, seeing as DRK seems to focus on parry, is to add a percentage of parry to their tanking stance. 15-20% or something of the like. The only other way I see of making DRK truly viable is to take the CD off Reprisal and make it a proc similar to Inner Beast on WAR.