That, in and of itself, isn't an impediment to a movie being good. Plenty of adaptations of books have thrown the book out the window and done a decent job. The problem with World War Z is that it is an action movie where the action isn't terribly interesting and a horror movie that isn't particularly scary. It is even difficult to write about because it is so bland, like a slice of Wonder Bread and a glass of milk.
The film starts off with some promise. It doesn't spend a great deal of time on set up. We meet a family; dad has apparently quit a job recently working under circumstances that are dangerous. Then they are in a car in Philadelphia and shit goes crazy. Zombie-attack crazy.
These are non CGI zombies and we see people getting bit and turning and then Brad Pitt's Gerry burns rubber. There are scenes of civic mayhem and conversations of rescue which happens shortly (as you might imagine, not without incident).
Brad Pitt and everyone else in the movie are fine but no one is given much to do. They seem to think that talking is bad and once there is any hint of any actual plot development there needs to be a large-scale CGI zombie attack.
Once on the ship the story and problems start to pile up. Gerry's family is on an aircraft carrier run by the military and UN. And he is sent out on a mission to discover the origin of the zombie plague. Basically from here he flies to Korea. Then decides to go to Jerusalem and finally to Wales. No need to refuel the plane, of course. There are, naturally, zombie attacks in all these places. It is very formulaic. Arrive in new place and then zombie attack. Hell, get on plane, zombie attack.
There is no chance in any of this to give half a shit about any of the alleged characters.
Next, when you find non-CGI zombies they are more comical than scary. The snap their teeth together in a motion that, presumably, indicates they want to bite you! OOhhh...scary. It is actually closer to laughable.
And with the PG 13 rating it isn't very gory (I am sure it will come out with a gorier version on DVD).
Another problem with this movie is its lack of attention to detail. They send Gerry on an extremely important mission and give him a satellite phone. The assistant secretary of the UN doesn't seem to have a cell phone, nor does the military commander (who, oddly, appears in one scene). Who gets the other satellite phone? His wife. When he calls in with important information she has to run with the phone. This is, of course, just an example. There is really nothing to this movie. It is all air and CGI.
More egregious is how this is obviously just a set up for sequel. Hopefully there will not be one. The resolution is sort of clever (and again has nothing to do with the book) but it isn't done particularly well. It sort of just ends and ends with a lame voice-over no less.