If you're rating this by Heroic Progression tanks, then I disagree completely. Monks are pretty much must have because of the insane damage they do, on top of the absorbs they put out, and then they have all those niche abilities to top it off. And then Paladins come after that with alot of damage, alot of off healing, and also alot of utility.
Warriors are good tanks for the sake of tanking, but there are other classes that can tank well and then some more.
On topic though; DKs are very viable for progression, don't let anyone tell you otherwise. They do require quite a bit of skill to minimize your damage intake, as you are in charge of keeping yourself alive. You won't do as much damage as other tanks, but there are a few small but noticable things that are nice for DK tanks, such as high vengeance blood worm explosions that can top off a raid.