I like it!
It doesn't bring much new to the zombie genre, but then again, if you execute something this well, innovation doesn't really need to factor in. I've already signed up for the email list for the Dead Frontier MMO. I've played all three of your mini games and they've gotten progressively better. I can only imagine how great the actual game is going to be, as well as what kind of stories will unfold within the quest system. I'm totally looking forward to attempting to survive against the zombie hordes with my friends online.
I really can't complain about anything here. I liked the graphics, the engine, the controls, the added companion characters, the sense of urgency you managed to convey, the enemies were well done, and the last boss was damn chaotic with that timer going.
Good work, man.
Keep it up, please.