Took DRK for a spin, and it kind of clicked what doesn't work at all with them.
1) TBN is strong. TBN is great when fighting in trials and end game raiding. TBN sucks when you are dealing with multiple angry midgets with hammers smacking your knee caps while double pulling packs of mobs in dungeons, which is why I feel like it basically doesn't have TBN during dungeon runs.
2) They got WAAAAAAAAAAAY too many OGCDs. There's no way a drk can spare enough OGCD slots to blow enough cooldowns to survive some scenarios while also using the same OGCD slots for half the stuff they got to throw out. You're either firing everything on full cylinders like a DPS or your trying to dump enough cooldowns to survive a major tank buster.