I don't think it's exactly unique so much as an excellent amalgamation of different zombie stories. The things that come to mind are the myriad of different types of zombies (Resident Evil), the diary style flashbacks (Day by Day Armageddon series), and the introduction of "smart" zombies (Land of the Dead). This is by no means a slam towards the writers, seeing how the storyline is expertly crafted. With so many different stories about zombies out right now it is almost impossible not to have some kind of overlap with some of them. Unlike a lot of zombie fiction WA has some excellent writers and voice actors that add a lot to the story. The continuity of the story and attention to detail are top notch.