All good things must come to an end, and so it will someday be with WoW. Whether in three years or ten, someday the servers will close, your characters will be deleted, and Azeroth will cease to exist. It's going to be a scary time. I can imagine hordes of people emerging from their homes, blinking in the sunlight, only able to communicate with crude language such as "lol nub" and "water plz."
Deadhoof of Thunderlord has already thought of that day, and he's wondering what the final boss is going to be at the end of WoW. We've already faced Kel'Thuzad, and we'll be seeing Illidan soon enough -- who is Blizzard saving for last? Who will be our Bowser, our Ganondorf, our Whatever The Bad Guy Is In Final Fantasy Nowadays?
Sargeras (the head of the Burning Legion) is the first name that comes to mind, but killing Sargeras will effectively end the Warcraft mythology, and Blizzard probably wants to keep the door open for the future. They've already said that we'll have to be around level 90 to kill Arthas, so he's a conceivable final boss if WoW only lasts for a few more expansions. Neltharion/Deathwing is always a possibility as well, but he was always a sideline to the main WoW story and thus unlikely to be the ultimate boss. My favorite suggestion comes from Gayleforce of Kel'Thuzad, who suggests that Blizzard pull a Time Magazine and make the last boss a mirror: "It's you! You are the last boss in World of Warcraft!"
If I had to bet, I would bet on Kil'Jaeden. He's a major figure, but his demise wouldn't totally destroy the story -- Sargeras can always find new lieutenants. Who do you think we'll fight at the end of Azeroth?