Popular MMO blogger Tobold is back from a short hiatus and asking this very question in order to disover the basic elements and motivations that make us choose to play these types of games. His introduction piece talks about some of the components he may cover, including: storytelling, character development, social interaction, polish, challenge, and achevements.
Since the introduction piece, he has written two articles in the series: Storytelling and Gameplay. Based on the quality of stories found in MMOs, tools and design iterations that all but allow players to skip over quest text, and the fact that pure MMO environments don't facilitate epic storytelling very well, he doesn't feel as though storytelling is a major reason for why we play MMOs. With a little effort (read: a lot), it could be. As for gameplay, he believes that unless new quality titles start releasing with gameplay that diverges from basic hotbar button-mashing combat and simple crafting, MMOs will be forever stuck in a rut and compared to World of Warcraft.