The plus you get out of a DK is that even in a tank position he can put out loads of damage compared to the other tanks which pale in comparison to the DK, I am of a mind to level one to try it out and see how I like it. I currently have my Pally (actually 2, first one I got to 70 then started another since the 70 was a healbot, the second one got to 80 a while ago now my original pally is 77) and I enjoy him very much as a tank, but DK would probably down crap fairly quick but cant complain too much my set up of 3 shaman, priest, pally take mobs and bosses down quick. Now second advantage of 4 shaman is that you can make them all elemental, well at least when you get the DK to the 30k health and max defense then you rarely have to heal him, you seen some of the videos they have out there, the dps of the shaman combined with the DK tank is crazy. Now with 3.1 coming along you get even more benefits from running shaman they will be buffing CL and will be on a much shorter CD so you will be able to use it more often.

But always remember go with what you have most fun with, if you just don't enjoy the shaman dont waste your time and go with the mix group.