Zombie Rule #1 says that if I shoot a zombie in the head it dies. To make combat fit into the types of progressions that are expected in an MMO, you may have to throw the head-shot rule out ... which sucks. One-shot location-based kills that are independent of monster level just don't fit into DIKU power progressions.
Zombie Rule #2. In a zombie movie, if you get bitten, it's game over. You have a few hours before the zombie bacteria kills you, and then you come back as a zombie. No-one ever survives a zombie bite. You don't "get better". So, if you are making a zombie MMO, you have to have an answer for his too (probably some kind of cheaty vaccine).
What's My Motivation. If I'm a character in a zombie movie, my motivation is survival ... pure and simple. In a zombie MMO though, what does my character aspire to? What's the item/skill/ability/achievement/thingy I can achieve or acquire that's going to keep me playing for a few more days, or a few more months. What are the carrots? Guns and the zombie rules above can be overcome with some hand-waving. However, what the player's motivation is is the most important question that must be answered when making a zombie MMO. You need players to keep playing (and paying).
And there's no easy answer. Characters in this genre do not progress. They either die or they don't. The motivation is survival, not a slightly better shotgun.
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.
There are certain rules that must be considered and certain experiences that are crucial to the zombie "feel". If you don't stay true to your source material, you run the risk of not delivering the zombie experience that zombie fans want and expect.
If Undead Labs have cool groovy answers to the motivation question and how to retain the "zombie experience", they'll do fine. I'm sure they do. I can't wait to see what they are going to do with the genre.
So, how else could you make a zombie MMO? Hmmm. I think what you don't do is try to turn Left for Dead into an MMO. While that frantic in-your-face bullet festival is a fantastic game and zombie experience, I'm not sure that you can build an MMO out of it, and expect players to stick around after the novelty has worn off. It's great lobby-based "sport PvP", but not really an MMO.
Instead it must be a deeper experience. I think a cross between Day of the Dead Online and Zombie Tale in the Desert is a good possible way to go. To keep players involved and motivated, I think you have to build a strong sense of in-game community. Accentuate the player's role and his or her worth to others, not his or her level. For example, have players learn jobs, such as medic, bullet-maker, gun smith, mechanic, chemist, etc. Have players work for their survival. Create lots of places that groups of players can turn into fortified camps. Have lots of player camps scattered across the world and have camps compete over resources. Make the camp the primary measure of power in the game. Make progression about what you can do and make instead of how powerful you are.
When players want to go outside, then go all Left for Dead on their ass. Create some systems that encourage players to travel to other camps. Evoke the road trip feel from Zombieland. Make progression about what you can do and make, instead of how powerful you are.
Oh, and you absolutely have to allow players to play as zombies.