As I - much to my surprise - enjoyed the book, it would have made sense not to watch the film. I wasn't going to. Then I don't know what happened, but for some reason (as you may have guessed by now) I did.
This is what is wrong with it :
1. The global, somewhat impersonal nature of the novel is gone, to the benefit of a fairly ordinary story about a family man fighting to save his female, hence helpless, family.
2. Zombie-attack scenes galore! Way too galore! Even squeamish me ended up numbed to the violence after a while, and I generally squirm at fist-fights. (Less is more, as a general rule...)
3. The Pitt-character could well have been played by Tom Cruise, to very much the same effect; that is how 100% invincible he is. Which is all the more inconsistent as the whole point with zombies is that they are totally unstoppable.