What sets MMOs apart from other games are their social structures and communities. If you just want to kill things and level up a character, you can play Diablo. If you just want to wander in a 3D world, you can play Morrowind. If you're simply interested in fantasy or sci-fi, there are accomplished narratives like the Baldur's Gate series or Knights of the Old Republic. But what those games can't offer is a role in the complex and evolving set of living social structures. In MMOs, you'll find guilds, role-playing servers, warring factions, economic classes, crime rings, griefers, political groups, and even entire cities and nations.