Untitled Zombie MMO
From My wiki
What we know so far
Undead Labs currently seeking a partnership with an unnamed company. An announcement of the partnership has been promised "soon" and are currently focused on staffing and refining core game design.
Class 3 going to be released at first for Xbox 360 as a downloadable title(with Class 4 possibly on the Xbox 720) It was also announced a later release for PC.
According to the Rude Q&A it will be subscription based and will probably not have micro-transactions.
Undead Labs is expecting a game rated M, and will not 'sanitize' the game for a lower rating.
The Zombie MMO will be a console based massive persistent world with a large player population dynamic. It will be a "true server-hosted world supporting millions of online players".  The game is envisioned to have the ability for players login in on one console quickly and seamlessly and have console split screens intelligently, thus making it more accessible as a party game where players on the same console can do missions together. 
The game will depart from tradition MMO mechanics of selecting a target and having a hotbar of abilities with cool downs, instead it will feel like a traditional action console game where as Jeff Strain put it "pushing the X button will swing your weapon, and there is more physical movement throughout the game world" and "expect to be able to pick up a golf club and start swinging it" . It was also said combat will be very visceral and "in our face(s)" with "crowbars, chains, spoons, and, well, banjos."
There is a quick Glimpse preview available on youtube 
Players will assume the roles of survivors, but Jeff Strain has said he would like to see players as zombies happen. At least part of a player progression will be rebuilding towns and fortifying them.
In State of Decay players won't have characters customization, but rather a Community Customization. There are going to be 20 major characters, comprehensive of a full background, and 30 minor characters. If all characters are killed, the system will spawn a new survivor with some abilities. "Class4" will probably feature Character customization.
“Will there be cars, motorcycles, and trucks? Of course. Should you be able to drive them, crash them, and splatter zombies with them? Yes, please.”
Object in game are supposed to be movable and will give the player the opportunity to set traps for zombie and control their traffic.
The interface will be more transparent that PC based MMO's and is said to “get out of the way” of the game world. The UI will be leaner, lighter, and less intrusive.
It was said in a interview that "Keyboards don’t mix with consoles, and we’re not going to pretend that they do", instead Undead Labs will concentrate on Voice and Controller based communication. They plan on implementing some sort of chat filtering system.
No real information has been revealed yet but in a Eurogamer article Jeff Strain said,
I'm not sure there will be, other than if there are times when players can be zombies - there may be some PVP there. The fundamental conflict in this world is humans versus zombies, and I don't see us violating that in any way.-Jeff Strain
The game will take place in Post Apocalyptic world where society has collapsed. The game revolves around the players having inherited the Earth and they are restarting society by fighting zombie and recovering what was lost like the player's home town.
Jeff Strain in an interview has said that he finds fast zombies scarier than slow zombies, but a zombie MMO can incorporate many types of zombies with different behaviors. In a G4 interview he mention that we would likely see both fast and slow zombies in the game.