I think the reason it is so immensely popular is because it adapts to different playstyles that users want to experience in the game. The term "easy to learn, hard to master" comes to my mind. Everybody can enjoy their way leveling through the game, whether you go on your own, or party up with friends and strangers. And while one can argue whether it really takes a lot of skill to play in an endgame raiding guild, you cannot deny those people spend a good amount of time into getting every bit out of their character, while others play only one or two evenings a week slowly leveling their character or going to a dungeon in a random 5 man group. And then there are players who solely focus on PvP and Arena.
It's the same game, but everyone in these groups plays the game differently, and all of them have fun doing so. This is what makes it a fine game for me.
It's the same game, but everyone in these groups plays the game differently, and all of them have fun doing so. This is what makes it a fine game for me.