Druids are the most versatile class in the game, but they lose their instant versatility already on lower levels, as later on a druid that is specialized in the Restoration talent tree for example will not be able to effectively tank in a high level instance just by shapeshifting into Bear form. However, with the proper talent specialization via Feral/Guardian/Balance/Restoration affinity they can partly take role depending on a chosen affinity for a short time, until a problem in group is solved, like resurrecting a tank or a healer.