Secondly, the game feels good. By this I mean the movement and responsiveness.
Third, the game in its original form was particularly immersive as the WoW developers eschewed the loading screens that were an immersion-breaking feature of the big competitor at the time (ie Everquest). Sadly this design principle has fallen by the wayside in recent years.
Lastly, WoW came about at a time before social networks when online games were the first proto-social networks for many, many people. Through guilds you found likeminded people.