I've been running 4 DKs/priest as one of my groups (have a pally and a war tanks as well). I think of DKs as the unholy pallies (especially if they're 'unholy' spec) Initially I was running the tank DK as frost as everyone said that it's the tanking spec. After I speced all of DKs unholy and tried it again all I gotta say is wow! Holding agro is not an issues, dps is insane at this level (was getting 700+) with packs of mobs at 60!

One thing to see is how they will scale at 80. Spell rotations are longer on DKs and the need to target/click DnD aoe spell takes a bit more work but it's nothing game braking. After a few pulls you get into it (even with DnD x4). For now I look at damage mitigation in this order war->pally->DK (never played a bear so don't know that one). For 5 mans/heroics/10mans? I think DKs should be fine. Most of the mitigation frost talent seem at the bottom of the tree so for now I'll stay with unholy. Thought Hungering Cold is a good @#$ button if the mobs go after someone else and you need time to grab them back.