The Standard of MMO games.
Approaching it's 6th year since release, it is still the standard of what all other MMORPG games are measured to. Whether it is graphics, artwork, quest/leveling system, classes, etc they are all compared to the standards of what World of Warcraft (WoW) set and continues to set.
As it continues to spurn out expansions, the game thrives and is becoming even more popular as the years go on. The ability to draw in the casual, first-timer players to MMORPG is a testament on how great the appeal of the game is.