11.5 million subscribers and growing

There's a reason why this is the most played MMO RPG of all time.  When my wife (yes my wife) convinced me to start playing I was skeptical.  I'd play D&D with my older brother a little when I was younger thought that WoW would just be put to shame by its simplicity.  Boy was I wrong.

The original map of Azeroth (the world in which WoW takes place) is absolutely monstrous.  If traveling only on foot and boat it would probably take someone the bulk of their entire day (maybe even longer) just trying to discover each area.

To keep people interested and leveling up NPCs (non-player characters) give out quests (literally thousands of quests available).  The quests can be as simple as telling you to go talk to another NPC, or telling you to enter a Raid (dungeon with many, many other players) and kill a boss.  Many of the quests get extremely creative and ask you to gather materials, kill monsters, find hidden objects, etc.

Upon hitting the max level I thought I would be "done" with the game.  On the contrary.  This is just where the game finally begins.  Once you reach max level there are more dungeons to run (with 5-25 people) with varying degrees of difficulty and all kinds of rare and epic items that drop.

Different gear has different attributes set on it.  Some gear might add strength, stamina, attack power or agility if you're a physical/melee class like a rogue, warrior or hunter.  Other gear might add intellect, spell power or MP5 (mana regen every 5 seconds) if you're a caster like a warlock, priest or mage.

Each class has a slew of abilities and ranks of those abilities.  The higher your level, the more abilities you get and the higher ranking those abilities become.

Upon reaching level 10 you begin to learn talents.   You choose how to allocate those talents, making certain abilities even better, and sometimes the talents themselves are new special abilites.  For example, not all level 80 warlocks are the same.  One might be specced deep into affliction talents, allowing him to apply several damage over time spells, while another warlock might be deep into demonology, giving him a new pet, or even the ability to turn into a demon himself.  Still another warlock might be destruction specced, giving him huge burst damage.

This only skims the surface of the World of Warcraft.  Races, classes, talents, loot, mounts, bosses, NPCs, cities, mobs, etc. are all so vastly different that many people are able to play this game for hours on end every day for years and still not get bored.