MP management seems to be the class' biggest problem at least in FL, even getting chipped by someone with something as measly as an autoattack means you absolutely have to drop Darkside if you aren't going to be seeing combat for awhile. WD I don't see it being as big of an issue due to the more fast paced nature of it, but its still there.
I haven't noticed a lack in defense- Shadow Wall definitely isn't Sentinel and parry is bad but I feel Dark Mind helps the class out a lot more in PvP due to its cooldown; often times I'd be able to use Shadowskin/DA+DM/Grit (if necessary) while pushing an objective and was able to pretty much laugh at any damage casters sent my way (and laugh even more if I was able to hit a group of them with Carnal) and get out of trouble before things got hairy.
For melees, Shadow Wall works just fine and I love to combo it with Bloodbath+Push Back, especially during a push, and their LB is no more threatening than it is to the other tank jobs.
Carnal Chill is amazing for nuking the damage dealt of priority targets or people in full burst mode and has a fairly decent AoE, can work very well in conjuction with the other tanks defensive PvP CDs, but it does suffer heavily because of its situational nature. Shield effects (Stoneskin, Galv/Noct shields, DV) kind of kill off its AoE aspect though (it won't spread if it hits a shield)
Tar Pit is good for making sure priority targets trying to get away don't get away, or can be used to slow an incoming rush. Of course, does this justify the loss of a knockback skill? I can't really say since I haven't used it enough to compare, or rather, I don't use it enough. I could probably live with it being replaced, honestly.
So yeah, I haven't really had as bad of a time with DRK as I've been hearing from some. The only thing that really bugs me is the occasional MP issues in FL, and the animations. The class has great bursting potential for a tank, but the animations on things like C&S, DP, arguably Souleater too can make it difficult to secure kills with friendlies around, and is especially annoying against healers. Too often I've had times where a healer was pretty much about to die only to be saved by a clutch heal and C&S's lengthy animation. Oh, and the fact that our stun is the worst overall in PvP (unless, you know, you have Dark Dance up and get insanely lucky as someone hits you)
Other than that, it feels a lot better to transition to after having played through nearly all of my lv50 PvP time as WAR. I don't think I'll be using it in WD though.