Given the breadth of user-created interface add-ons available for World of Warcraft, there simply isn't any reason for you to be running the "stock" version of the client. While Blizzard does its best to adapt the game's default interface to the innovations created by the hundreds of add-on developers on the scene, it's ultimately a game of catch-up the company inevitably loses. Simply put, you aren't getting the most out of your game experience if you aren't running some key add-ons, and some of these have become so essential that many guilds will out-and-out exclude you if you don't get with the program.
However, sifting through pages upon pages of add-ons at the various clearinghouse sites is a daunting proposition indeed. This is why GameSpy put together the following list of essentials. If you've been out of the mix for a minute or a year, this list of mods will get you back in the game and raid-ready in record time.