Some people just don't get MMOs. 'What's the point? You just keep leveling up, but there's no end to it. Where's the story? Why would you pay a monthly fee for the chance to keep getting killed by the same griefer over and over?' Of course, what they don't understand is the feeling of community, the excitement of content updates, and the chance to partake in an ongoing history that's being partly written by the players themselves.
But at some point, the heretofore non-MMO gamer decides to take the plunge, whether out of curiosity, or the constant browbeating of his more evolved peers. He might give World of Warcraft a spin, considering how popular it's grown. Or he'll jump into City of Heroes out of love of the comic book medium. Or, being a Tolkien fan, he'll download the Lord of the Rings Online trial. Suddenly, the scales fall from his eyes, he renounces all worldly goods, and becomes one of the converted. Oh, he might still play the occasional single-player game, but he'll never give up his guildies.
Which MMO made you see the light?