The characters are so similar and so lacking in backstory and identifiable characteristics
How is it problem?
This seems to me to be one of those rare instances where the movie will probably be better than the book.  As many others have mentioned, this book starts very strongly.  The first several chapters focus on modern-day (or near-future) characters, and the writing is so strong that I actually recommended the book to several friends before I got to the next section, which takes place about 100 years in the future, after the vampires have been released on the world.  Once that shift has taken place, the book drags.  The characters are so similar and so lacking in backstory and identifiable characteristics that I had trouble keeping some of them straight.  On top of that is the fact that there is no resolution--the book just ends, in a way that makes it clear the story will be continued in a sequel.  I hate when books can't stand on their own.  Nothing on the jacket made this book seem like the first of a trilogy, so the lack of an ending to a quite long book was very disappointing.  And one thing that kept bothering me throughout the book that made absolutely no sense (and that I hoped would be explained) is the scene where one of the main characters, Amy (a young girl who becomes the "zero"--the first success in the viral testing that inadvertently unleashes the virals--or vampires--on the world), is taken to a zoo.  This is well before she is subjected to experimentation.  At this point in the book, she's just a normal little girl who has been abandoned by her mother.  But for some reason, every animal in the zoo goes crazy when she walks in, attacking each other and creating so much havoc that some of them have to be shot and the zoo has to be evacuated.  Like EVERYTHING in this book, this scene was described in very cinematic terms, so you can see exactly what the author thinks the movie version should look like (the movie rights have already been snapped up by director Ridley Scott's production company).  But, regarding that scene: it makes no sense at all.  Amy tells someone that the animals are reacting to her that way because, "They know...what I am."  I have no idea if this will be explained in the subsequent books, but it bothers me still.  This was really 2 1/2 stars, but I'm rounding up because of the strength of the early chapters. 