actually for new comer I would recommend paladin/shamans, instead of dk. Paladin can easly hold agro with one macro and gives very nice buffs to the group and has bop.
DK on the other doesnt have the threat generation of a paladin, the only buff that it gives to the group is ebon and threat generation is much slower then a paladin, gets even worse if You use a macro to do it, as Dk rotation is more complicated, and full rotation consists of doing abilities in order for good 20s(goes like this IT,PS, BS, BS, UB, SS,SS,SS, DC,DC and as tank You want use RS when its available)
Paladin atm can do double shield slam then pick his nose, as other wont pull of him no matter what:P
I have both tanks at 80, I like DK more for the avoidance part, and how much more complicated they are.