You can't enjoy much of World of Warcraft's end game unless you join a guild. There are many different types of them - some easier to join than others - with different requirements, expectations and environments for play. Many people switch guilds multiple times because their old ones fall apart or they want to do something different in the game than they have been. The higher end guilds require applications as involved as if you were trying to get into an exclusive university or land a nice job. Expect to go through multiple interviews and to have a prepared "gaming resume" if you are trying to get into one of these exclusive organizations.

If you are just getting into the game, it makes sense to get into one of the more in demand classes than one that is highly competitive and in low demand. For example, a guild only needs at most two or three warriors at one time, making it a crucial but competitive slot. Healers are less common, but even so guilds need only a few of each healer.

With the way raids are structured since Burning Crusade, guilds need a variety of damage dealing classes in order to succeed on a variety of encounters. No guild can succeed if they stack mages, rogues or DPS warriors, for example - they need a good balance. Most importantly, they need skilled players that are emotionally stable - far more difficult than you might think to find. Maturity is in high demand, nearly as much as an ability to show up to multiple raids a week on time.