Starting today, October 15, World of Warcraft basic pack now includes all the expansions up to Cataclysm. All the accounts that were a bit behind were upgraded as well to include the Cataclysm expansion. So if you are a subscriber and your account has only Wrath of the Lich King, now you have access to the Cataclysm features for free.
So, players are able to get the original World of Warcraft, The Burning Crusade, Wrath of the Lich King, and Cataclysm for only $19.99. The upgraded game pack is already available in the Blizzard store as a digital version and the retail packs are going to reach the store shelves pretty soon too.
Now if you ask yourself what expanions do you need to be able to play World of Warcraft the answer is simple: you only need this new pack and after you are getting close to level 85 you can purchase the Mists of Pandaria too.