The Spider-Man, Superman, and Batman movies that work well, do so, in part, because those characters are all brilliant creations, in their own right. Even last year's Fantastic Four movie, which doesn't even try to achieve 1% of the visual extravagance and elegance of Jack Kirby's comics, gets by on the strength of Kirby and Stan Lee's original characterizations. Stan Lee, Steve Ditko, and Jack Kirby had a gift for coming up with these explosive pop culture concepts. (Shuster and Siegel and Bob Kane and Bill Finger aren't quite giants, in that they more or less stumbled upon their great creations by accident, but they did manage to tap into some very powerful mojo). The movies based on these comics tend to be good based on the extent to which filmmakers can translate the basics of the concepts to the screen.
The thing with Alan Moore is that he's not at all "creative" in the same way that Lee, Kirby, and Ditko (and others) were (or even in the same way that Neil Gaiman, Frank Miller, or Grant Morrison are). Moore doesn't create his own powerful, potent, pop culture concepts: he takes other people's concepts and turns them inside out or rearranges them or makes them answer all those unanswered subtextual questions that, not coincidentally, helped make the concepts powerful and potent to begin with.
He does it in From Hell with the Jack the Ripper created by fringe Ripperologists, in Whatever Happened to the Man of Tomorrow? with Mort Weisinger-era Superman, in The League of Extraordinary Gentlemen with late 19th Century pulp/adventure/genre fiction, in V with ubermensch mystery men-type characters like The Shadow and Batman, and in Watchmen with Steve Ditko et al.'s Charlton heroes.
But when Movie Industry People look at his comics, all they see is the potent, powerful, pop cultural concept, and they ignore all the other stuff, i.e. the actual "Alan Moore" stuff. The way Movie People use his comics is a lot like the way they use Philip K. Dick's short stories: to provide a clever hook on which they can hang a standard action thriller.
