You can always go back and start an alternate character (aka alts). Lets say you want to be a Tauren instead of an Orc for once. The thing is this. Been there done that... The last thing you want is a game to become a chore to play.
Here comes the Expansion! This is where they re-invent the game and package it and sell it to the 12 million people playing. When you consider that this particular form of entertainment costs less per month then any other entertainment offering by an outside supplier (yeah, lets not split hairs here). A redo of your virtual world can be a fun time.
So this December the World of Warcraft is releasing Cataclysm. Where a heretofore presumed dead bad guy (bad dragon) rises from the ashes and opens a can of whoop ass on the planet of Azeroth. Nick named Death Wing. He does what super-villains attempt to do, mess up the world pretty badly. We heroes now have to make it through the maze of storyline and finally confront him.
Back in 1994 Blizzard came out with a computer game called Warcraft: Orcs and Humans. This was a strategy game that laid the foundational back story for the World of Warcraft. They released Warcraft II in two parts, then Warcraft III in 2002.
The writing staff for Blizzard went in depth with multiple story lines and intriguing characters and history. This rather rich tapestry has been laid out in a number of Books, Graphic novels and of course, in game.
When World of Warcraft entered the market in 2004 there had already been several successful MMORPG games, they took the best aspects of the successful ones and built upon that adding a remarkable lore.
Imagine, if you will, a favorite work of fiction and you can suddenly interact with said book on a personal level, with friends. The immersion effect is quite overwhelming and very cool.
The Blizzard folks do not take themselves all that seriously and pop-culture references abound.
So this next expansion is looking very fun and promising. (BTW I can send you a free trial of Wow if you would like)