1) Many different shotguns and shotgun modifications are a must. 2) It must be possible to become infected and play as a zombie though there should be a distinction between the type of zombie the player becomes and the mindless zombie mobs. Think the difference between the forsaken and the scourge in WoW. PK players that become infected could join a clan (or some form of group) of undead and there can be an uninfected group of PK players dedicated to eradicating the PK undead.
As stated in the topic title, what features would you like to see in a zombie apocalypse MUD?
Interesting that this topic came up, considering I've been discussing this idea with a number of people in the MUD community for the past couple months. Speaking of, do I know you by a different name perchance? If not, we should talk. I have a lot of the key components (already working) and designs for further game mechanics. E-mail me (email@example.com).
Shotguns can be fun, but I'd also enjoy an Army of Darkness style setting. So for me the only real 'must have', I think, is zombies.
Warriors: Typical warriors, class name can be something different, they get all the battle skills Medic: Can cure diseases and heal wounds, at later levels they develop skills to halt the spread of the zombie virus in mortals Scientist: Develops/Creates weaponry for the warriors to use, there are other possibilities here I'm sure but I can't think of them at the moment.
Zombie: Undead equivalent of the Warrior, they can infect mortals with the zombie virus Necromancer: Can raise NPC zombies to fight for them and can command already existing NPC zombies. Lich: Uses magic derived from the death of mortals, like the wizards of the undead. You probably already know what liches are capable of.
There's a few ideas for classes, they aren't complete but it's a good start.
You should make it so that a player doesn't choose class at creation, but through choices made in-game. A player goes through mud school as a normal human and at the end they have two doors to choose from. One leads to the mortal stronghold and the other leads to a room infested with zombies where the player is infected with the zombie disease. A PC zombie then can begin training to become a necromancer or a lich, or they can remain as a zombie. The same goes for the mortals, the player starts as a warrior but can choose to train with an NPC to become a scientist or a medic.
[edit to add] The NPC which a player goes to for medic training should offer his services to halt the progression of the zombie virus for a fairly high cost. That way players can avoid switching teams if they can afford it. If they can't afford it they'll need to find a PC medic who can do it for free. This encourages team-work.