I think the difference with this over a Dark Knight or Iron Man is this was more like an adaptation of a novel rather than a story about some comic book characters.
It'd be like doing adaptation of something like The Dark Knight Returns, in that there really should be as much of the original story included as possible, rather than writing a whole new story.
Iron Man and Batman are broad spectrum characters, who have changed a lot over the years, but Watchmen and Dark Knight are confined stories.
I think most fans of the comic book would be too pissed off if it was changed to much, just as any fan of a book by Stephen King, Dan Brown or whoever, would be pissed off if an adaptation of one of their books strayed too far from the source.
This film was certainly aimed at fans of the book first an foremost, where as most comic book adaptations are aimed at the non comic book reading public first, then moulded to make it acceptable to the fan boys!
