eh, some specs are horrible for DKs. what comes to mind for me is 2h frost dps. It just doesn't work compared to blood, unholy, or DW frost. And unholy presence is basically worthless, even in pvp. I mean it's kind of good to give yourself a speed boost, but it's such a huge dps loss that it's hard to justify using it, even in pvp. Not that it really takes anything beyond facerolling as a DK to win in pvp.
I mean tank-wise it's ok, but I guess I'm mostly looking at the frost tree and realizing holy shit, there's no way I can take the fun talents, but still get the good tanking talents without spending way too many points. although I don't really like any DK tree right now (not even saying blood or unholy are bad, I just don't really like the playstyle) and probably never will until they inevitably nerf ret paladins, because hybrids aren't allowed to have fun.
I mean for me, blood DPS is all right, but is basically a gimp ret paladin, down to the copied vengeance self-buff and crappy group healing from dps (in a different way than judgement of light). I also feel like it's pretty much mandatory to go with annihilation from frost talents or else you're gimped, and I mean from the instant you decide to spec blood. Frost 2h is tedious and not worth doing, even though it can be fun to see the absurdly huge crits from frost strike + cinderglacier sometimes (note that that pretty much goes away at 70+ because fallen crusader is a better rune). Unholy I've never liked, so I can't really comment on it.
I guess blood has more wiggle room so you can pick up good dps talents and good tanking talents compared to frost. Frost is so stupidly segregated and a lot of its dps talents just suck. They might as well combine stuff so you can reduce points on the tree, since I think that would be a better way to buff it than simply increasing the effect of underused talents, unless they can find some way to make frost into a melee mage.
The main thing for me is, as a pally player, I feel like pally prot makes me into a way better tank than a full on tank spec DK would be, if only because I can dish out as much threat but have a much easier rotation, and I can pick up awesome non-tank buffs (at least, not directly tank buffs) that most DKs could only get via runeforges that replace their new awesome/near mandatory tank rune. And likewise for ret vs DK DPS, with the added bonus being that divine storm will hit more targets than heart strike and then heal the raid, and I get to give replenishment, too.
I'm not even saying DKs suck; it's just that mine feels like my paladin, but not as good. And it's so similar I guess that's why I couldn't really play it beyond a few levels.