The old phrase "jack of all trades, master of none" is an excellent way to describe the Shaman; they are the only real "hybrid" class in World of Warcraft. While they do not excel at any one aspect of their repertoire, they function passing well when performing most tasks called upon by a party. Need a support healer?
Looking for a secondary tank?
After another damage-dealer? Have an empty slot that needs someone to support the group in general? The Shaman can provide. They have a number of utility spells as well that help make life easier for any party.