I don't read a whole lot, so I probably just don't understand, but why do movies have to be exactly like the book they are based on? Why can't they just be subjective takes on the book? I just don't understand why people can't appreciate a great movie because it wasn't 100% spot on with the book. Just my $.02
Just my personal opinion, but if you read one story, and then see a movie of the same exact name, with that story's author's name attached to it, then you have some expectation that the movie you are about to see will be at least loosely attached to the story you read. World War Z the novel was a fantastic read.
Now, imagine you read How to Brew (I know, this is a stupid example, but stick with me for a second), and really dug it. And then a few years later you saw that How to Brew was being made into a blockbuster movie, with some involvement from Palmer. Then you went to see it, but instead of being about, well, how to brew, it was a romantic comedy in which the male star happened to drink a homebrew in a few different scenes, and maybe a fermenter shows up once or twice, but aside from that the movie has almost literally nothing to do with the book you read? You'd feel pretty let down, wouldn't you? You'd wonder why the heck they called this thing "How to Brew" when it bore little to no real relation to the actual source material.
Taking a few liberties here or there, I get completely. Take, for instance, what Peter Jackson did with The Lord or the Rings to shoehorn those tomes into three movies (and, I guess, to make a part for Steve Tyler's hot daughter). Or take what they did with the ending of My Sister's Keeper. Both were very true, for the most part, to the source material, but took certain liberties (in My Sister's Keeper's case, the movie eliminated a very important character and completely changed the ending) - but those liberties worked, and made the story work for the structure and time limitations of their respective movies.
World War Z, on the other hand - based on previews alone (and I understand those are limited at this point, so I grant I could be somewhat off base) bear almost no resemblance to the book, with the exception that both involve Zombies. But even on that count, the movie version of the Zombies bear little resemblance to the book version.
To be quite honest, the way World War Z is written, I don't really see a way it could be made into a movie... It's basically a book of interviews with survivors after the end of the apocalypse. Some pretty terrifying stuff.
Simply put, if you're going to create an original work of fiction - great, more power to you! The world NEEDS more of those! But don't create an original work of fiction and slap something else's name on it in an attempt to lure that something else's fanbase into coming to see your original work! If your original work is worth something, it'll draw people out on its own merit. Whether it is or isn't worth something, you're likely to irk that something else's fanbase that you lured out on basically false pretenses.