Sometimes I get the feeling that unsympathetic viewers will bring in a concept like "historical accuracy" to make their argument against a movie stronger: it gives the impression that they are holding the movie up to an objective standard. This is a suspect maneuver, IMO, because most viewers (general audience members, film critics, and film buffs alike) are not at all consistent in terms of which movies they hold to that standard. "Historical accuracy" is like a club they pick up when they feel the need to beat on movies they don't like.
So we can use "source infidelity" as a clubm, but also as a hook. That's basically what I was doing in my post on The Spirit. It isn't the fact that Frank Miller's movie isn't faithful to Will Eisner's comics that makes it a bad movie, but comparing what it does unsuccessfully to what Eisner's comics do successfully is useful and illuminating. More importantly, perhaps, it is a natural comparison to make.
We can also talk about different kinds of infidelities. The movies of Atonement and The Lion, the Witch, and the Wardrobe all stick reasonably close to their source novels in terms of incident, character, and theme, but fail to translate the sensibility of the original, because they substitute the conventions of contemporary, mainstream narrative cinema for the more personal voice of the respective novelists.
The Narnia movie takes C.S. Lewis' "stuff" and presents it as a souped-up action/adventure fantasy, with none of the fragility of the original story. Despite its heaviness as an allegory, I think that Lion shows a light touch (especially when compared to Tolkien's more elaborate fantasy novels). The Narnia books are more bedtime story than fantasy epic, something that the movie completely misses.
The movie of Atonement is high-energy and twisty-turny - a period melodrama played like The Usual Suspects. It's a choice that works (on a certain level), but it also jettisons the more contemplative aspects of the original novel (which happen to be those elements which made the book memorable). The effect is that the movie is more intense than the book, but also less gripping - or maybe less grippable: there's not as much there to think over once the movie's done.