Would like to add something.
Everyone knows it is true that there is a lot of chatter about how overpowered Warlocks are and that they need a nerf. Regardless of whether they do or not, a lot of people are saying it. That is a fact.
WoW's PVP is considered something like the Rock - Paper - Scissors game. Warlocks are often jokingly refered to as fitting in that scheme as WMDs or Napalm or whatever sounds the most overpowered at the time. Even Shmii's signature is a play on this. (It is from World of Roguecraftâ„¢ 2)
This part is arguable but many people will agree that Warlock is the scissors to Warrior, Rogue and Mage. That does not mean these classes cannot defeat a Warlock. It means that it is generally believed for Warlock to have an innate advantage versus these classes because of their nature.
A census for all PVP realms says that:
http://www.warcraftrealms.com/census.php?serverid=-1&factionid=-1&minlevel=60&maxlevel=60&servertypeid=2
Warrior, Mage and Rogue are the three most-played classes.