In many games, plot outcomes are affected by choices the player makes throughout the course of the game. This is obviously not the case in WoW -- but what if it were?
Currently there are actually a few decisions for players to make in the game. For instance, a quest in Eastern Plaguelands gives players the option of adding varying amounts of a solution to a plague cauldron, with different results depending on the player's choice. The option to associate with either the Aldor or the Scryers was a key dilemma players faced in The Burning Crusade. Similarly, in Wrath, players had to decide whether they would assist the Oracles or the Frenzyheart tribe, with each faction offering different rewards.
None of these choices influenced the unfolding story, however. So imagine how much more compelling it would be if the decision to support the Oracles or Frenzyheart tribe had some effect on, say, the Icecrown raid. What if the Oracles showed up and played a song on their instruments to paralyze the Lich King for a short time? And what if the Frenzyheart charged in and set traps that made him take more damage when triggered? Raids groups would then need to have at least one player supporting each faction in order to have them both show up.
Which storylines and events would you want to influence if player decisions really mattered? Would you have voted for someone besides Garrosh to become the new Warchief? Would you try to promote peace and cooperation between the Horde and the Alliance? Or would you pledge allegiance to the Old Gods and stymie the efforts made by other players to stop Deathwing and his ilk?