It is a (sad) fact that not everyone who appreciates great works of art appreciates them on all the levels on which they operate. That's an unwieldy way of saying that part of what I love about Alan Moore's work is that it is both cerebral and exciting - the Watchmen works as a costumed superhero tale at the same time as it critiques the genre. You can read it and think it's really great (and be right) while missing all kinds of extra subtlety. I get the impression that is sort of what happened with LXG - the film kept the adventure part, without exploring any of the nuances and references and esoterica that are really what elevate Moore's work above the rest. I am afraid the director didn't really 'get it' all the way; the bit made it clear he knew Moore was a hot property (based, I guess, on the buzz around From Hell - which he described as "phone-book like" in the first of what grew to be a flurry of thing that made me wince) and so he snapped up the opportunity to do another film of Moore's then-current project, which turned out to be LXG. I did not get the impression that he'd read or loved Moore's work, particularly and alas.
Which is why I decided that if I was going to watch the movie I'd do it before I read the graphical story. I like a good adventure flick - I even like a fairly brainless adventure flick, if I'm not expecting something more nuanced, at least. It was a perfectly adequate adventure flick, and if that is damning with faint praise, so be it. A little heavy on the CGI, a little light on the character development, but with tantalizing hints of things I hope will be elucidated in Moore's actual story.
I've essentially come to the conclusion that to capture what draws me to Moore's stuff on film would be very difficult. A lot of it is that delicate balance between action and erudition; a good portion of that even requires being able to re-read, or examine a page at leisure - it's hard to give the background details of a film subtle import because unless you focus on them (making them no longer background details) they go flashing past you at 24 times a second.
It's clear you can tell the same story on film as you can on paper, but it's always going to come out somewhat differently due to the nature of the media. It's not necessarily easy to go the other direction, either, and end up with a good novelization of a movie. In fact I can't think of any movie novelizations I've liked as well as the films that inspired them...