Well... I don't know, if it's just me but my first impression of the new DRK is ... really not that bad. I am saying that as someone, who came to FFXIV at the time of 4.3 and mains the DRK since the very first moment I set foot in Ishgard. In some raids in SB, someone told me that I have been a good DRK, so I guess, that I have been doing the SB rotation quite fine. I know, I was not in the pleasure of enjoying HW-DRK, but I guess to judge the ShB-DRK it is not that much needed.
I also need to say, that you guys have worried me prelaunch about the DRK.
Let's see...
My take on the MP-management is "Do I need to be ready for TBN in the next seconds or can I burn MP for my pretty offensive Floods/Edges?" I really don't care so much about keeping Dark Side up because it IS up all the time. MP regen is slower than SB-Bloodweapon or as SB-Grit-Syphon but it is not THAT slow to be ready for 1 flood/edge in 1 minute if you burned down your MP. I guess, the decision-making here is whether you need TBN in the next few seconds or not.
As for Abyssal Drain: I was worried, too, because it really was the sustain-aoe in SB. But my experience with ShB-AD is not that bad. I have been told, ShB-AD is doing 200 potency HP recovery only once per use. My experience is ShB-AD is doing 200 potency HP recovery for every mob hit by it. As I was leveling with FATEs, a big pile of trash mobs was charging at me. Because there was no healer with me, I just could watch my HP bar watch burning down. Then I used AD and BOOM -- my HP went back up significantly. From that experience alone, I am sure, ShB-AD still really is a thing. At least in aoe-situations. If THAT AD is a GCD, you will not need a healer anymore on trash mobs. You want some Clemency or Equilibrium for DRK? Pile a bunch of mobs on you and drain it out of them -- if you can tank them all that is.
And TBN itself is strong itself. 1/4 of your/your target's health as a shield. Everyone said, SCH -- the shield healer -- was the best healer in SB. DRK has his own shield. Use it to prevent damage instead of looking for something to heal you after you got hurt.