the different thing is not that but nice stuff are :

1-survival:having to take care of food,water etc..
2-coop can be fun really,but i imagined it would be better if we are playing coop vs. zombies,and no other online players...way more fun.
3-it's arma

things is this is still development,so not the end product for sure...
But,it's pretty close to what I would demand from a zombie survival game...atmosphere can be better.

what mods do you play?