The thing I remember most about my first reading of the book is that it defied all the expectations I'd had because it was a "superhero story." I wondered if perhaps the same thing might happen to moviegoers.
In retrospect, it appears everyone felt cheated. They saw trailers, and paid $10 to see a superhero/action VFX epic, and didn't get it.
The problem is the book is taking a genre which is native to the medium of comics and deconstructing it, often showing how silly the whole thing is and how daft the people feel in their costumes. However, superheroes rarely work in cinema, largely because films are generally stuck in the real world with real physics: It takes a huge amount of effort to create a world in which superheros become believable on screen, often taking liberties to make a parallel version that works for cinema (take The Joker's recent appearance for example). While the comic can just have fun knocking everything down, the movie has the job of first convincing you superheroes might exist in the first place, before knocking it down.
To tell movie audiences "everything you thought you knew about superheroes is wrong" is a risk: Unlike comic readers, it can't be assumed that your average cinema goer has much knowledge of, or opinion about superheroes at all. By selling it as a superhero movie, people enter into the spirit of it as a superhero movie (knowing also that it is not rated for children).
Besides, it is actually a superhero movie! If people feel cheated that it (gasp!) defied their expectations, it only shows the fickleness of movie audiences. I personally hate to see films in which nothing unexpected happens.