When dealing with a monster story, there are some rules you can break and some you simply cannot. Take vampires, for instance. If you can write a suitably snobby and dismissive blood-sucker, then I'll buy it when he or she walks down the street in broad daylight munching a clove of garlic. You're going to to tell me that werewolves don't need the full moon to transform? Fine, I can live with that too. But when we're talking zombies, there are two things which must be included: an undying hunger for human brains and a ridiculous number of the walking dead.
My hat is off to Mr. Grahame-Smith for being so bold as to take a beloved piece of English literature and force it into the same genre as The Night of the Living Dead. Indeed, the writing is such that a reader can be forgiven for thinking that the "unmentionables" were a part of everyday English country life. Each time Grahame-Smith takes a diversion from the original text, the transition is so smooth that, had Jane Austen been interested in zombies, I'm sure she would have written a strikingly similar novel. That said, I think the modern author has missed the very real fact that zombies are a singular instance of quantity equaling quality.
Have a look at movies like 28 Days Later or the glory which is Army of Darkness. Consider the Resident Evil video game franchise, one of the original digital purveyors of undead mayhem. What do they have in common? A fat lot of zombies, that's what! The reason these games and movies have been so successful, are so entertaining, is that a handful of heroes are faced with a whole horde of walking corpses, hell-bent (sometimes literally) on eating every brain cell they can get their decaying hands on.
As an antagonist type, there's not much to the zombie. He's got no personality which requires development and barely needs a back-story. You can make zombies out of nearly anything: radiation, misguided science, magic, you name it. Shoot, you can even get away without an origin story, provided there are enough hellions to cause a distraction. Zombies are wonderfully disposable, and that's what makes them great. In the case of Pride and Prejudice and Zombies, however, this essential rule of undead entertainment has been ignored for reasons I cannot fathom.