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MMORPG.com columnist Justin Webb writes this look at the idea of a zombie-based MMORPG, and some of the barriers that he feels exist between the horror sub-genre and MMOs.
The zombie genre doesn’t lend itself to being an MMO. The zombie experience thematically plays out more like Pac Man. You go around the maze running away from ghosts while gathering resources. Every now and again, you get a weapon that lets you fight back for a while. You keep running away from the ghosts, but they are relentless, and eventually you know that you are going to die. Pac Man doesn’t ever level up, or get new abilities, or an epic mount, or a purple shotgun. He always gets caught and eaten by the ghosts.
MMOs are different from Pacman in that they are all about player progressions. Players (arguably) expect levels. Players expect that their characters will progress and "get better". You can't just reskin a DIKU-style game and add zombies. You can't have a level-80 Shotgunner character with a purple Shotgun of Doom. It won't work. It will feel wrong. It won't feel like a zombie movie, which is why no-one's made a zombie MMO yet.