Paladins are probably the easiest tank to box since they can pretty much 1-button tank by setting up a castsequence for their abilities. Even if they lose agro, they have 2 taunts usable every 8 seconds, and a number of other "oh shit" abilities. They also have some nice buffs that synnergize well with shamans. A pally / resto sham combo can purge / cleanse most buffs / debufffs in the game.

Multiple shaman work well becuase of the stacking (healing, grouding) and complementary (resistances, buffs/debuffs) nature of their totems, wipe prevention and the abilitiy to both dps and heal (without sacrificing mitigation).

Cheers,
S.