The Shaman is the reason Horde melee groups out damage Alliance ones in Classic, full stop. Windfury Totem gives everyone in the party a chance for extra weapon attacks, and on a group of Warriors and Rogues that turns into a tidal wave of damage. A single Shaman dropping Windfury is one of the most valuable raid spots in the game, and it is Horde exclusive.
Beyond the totem, the Shaman is a self sufficient powerhouse. You heal yourself, you nuke with Lightning Bolt, you melee with Windfury procs as Enhancement, you cleanse, you drop totems for every situation, and if you die, Reincarnation lets you self res with mana and health to keep going. The class never feels stuck because it always has an answer in the totem bar.
The three specs all work in their lanes. Restoration is a strong raid healer with Chain Heal and the totem utility. Elemental is a bursty caster with huge Lightning Bolt and Chain Lightning crits. Enhancement is the melee Windfury enabler that hits hard while buffing the whole group. Whatever you pick, you bring totems, and totems are why people want you there.