I Was listening to a Filmmakers Q & A that Zack Snyder did on Playstation Home along with Dave Gibbons,and he essentially said that he is a fan of the Graphic Novel,and When Warners approached him to direct the film they wanted a PG-13 film that was possibly Franchisable,but he couldn't do that,it was either all or nothing hence why hes stuck to the more mature market
Im off to watch it at Manchester Imax in a hour or two,cant wait really looking forward to it