After an epic sea battle between the Horde and the Alliance, the survivors came across a land that had been lost. It hadn`t been really lost, the occupants, Pandaren had cut themselves off from the rest of the world. However after being rediscovered, the Pandarens set about taking on the evil in the world.
A new Pandaren would arrive on a floating turtle where they would quest until they`d have to choose which faction, Horde or Alliance that they wished to join. The floating turtle is inspired by the turtle in the discworld novels. One person spent over 170 hours levelling their Pandaren from 1 to 90 just on the island. The Pandaren is stuck on the island and won`t be allowed to get off until s/he has chosen a faction.
In previous expansions, a class would get the opportunity to do a series of quests for a legendary item, this expansion allowed everyone to try for the legendary item, a cloak. The player would do a series of quests for the Black Prince, a son of Deathwing, who you killed in the previous expansion.
Scenarios, three man dungeon instances which didn`t require a tank or healer were introduced and along with heroic scenarios later. However, they were dropped in the following expansion, Warlords of Draenor.
Championing, where you could wear a tabard for rep was dropped for all new factions except the Horde or Alliance`s Pandarian faction.
The main antagonist in the storyline was Garrosh Hellscream, who was the Horde`s faction chief. He was so bent on destroying the Alliance, even the Horde disowned him. Garrosh set about finding a buried weapon that would he hoped kill the Alliance but the weapon was so much more that everyone had to stop him. Garrosh holed himself up in Orgrimmar which both sides attacked and defeated Hellscream.