Tank - Paladin, Warrior or Druid
Healer - Priest, Resto Shaman, Resto Druid
DPS/Crowd Control - Mage
DPS - Warlock or Hunter or Mage

It is recommended though if you want to do heroics that you 5-box or run with a friend.

Controlling multiple classes is fairly complicated though. So don't say you weren't warned ;-)