why compare the book with the film
As we're all aware, filming a book isn't easy, they are two different formats with their own grammar.
I don't think this was a bad film, I think it's quite the contrary. I don't deny that it could have been greater, but then again, a lot of great films could have been greater.
The most important thing here is that the film held true to the original idea of the writer of the book. They didn't betray his 'voice' you see. Of course they had to scrap scenes, characters and whatnot, but remind yourself that it is virtually impossible to film a book exactly like it was written. Unless it was a book written with a clear goal of being filmed.
The whole idea of the film is to spread this message Thompson has about the American Dream. And I think the film succeeded in this. You can clearly feel Hunter's frustrations and impotence towards the powers of rich criminals.
So stop complaining, in stead you might consider to write a story or screenplay yourself if you think can do it better :)