When you think of the priest in the context of traditional MMOs you think of the token healer, usually part of the holy trinity along with the warrior and mage to get power leveling. Many players who picked the priest as their character when they started the game had the expectation that they would be the premier healer, playing the role of support by World of Warcraft Gold.
However, unlike in other RPGs we have tried to make healing a role that many classes can fill. This is why sometimes priests can feel that they are not balanced correctly, since they are not necessarily the best healer.
The priest is not a stronger healer than the other classes, but does have unmatched versatility. At its core the priest has two unique talent trees for healing, while the others only have one. Furthermore, the priest has strong heal-over-time spells (HoTs), direct heals, and area-of-effect (AoE) heals.