Kim Thompson says that The Spirit is bad in a way that suggests Frank Miller made exactly the movie he set out to make. I think he may be right, but a big part of what made watching the movie such a disappointing experience was that there were hints - around the edges - of what a better Spirit movie might look like.
Specifically: in the opening and closing voice-overs, the Spirit rhapsodizes about his beloved city, but the city plays no part in the movie. Where the Gothams of either the Burton or Nolan Batman movies are fully-realized places, Central City in Miller's movie is barely even a background: to call it abstract is giving it too much credit. Calling it generic, even, implies that there's some form there that's fulfilling certain conventions, but there's just nothing.
Anyway - that absence points to how Miller should have made this movie: with Central City as the main character and the Spirit operating in the background. Hey - he could even have split the movie up into three separate stories - maybe even stories from Will Eisner's comics - with some recurring characters to give us a sense of a living city.
For me, it isn't a question of whether or not Miller should have been more faithful to the original comics. However, completely ignoring what made the original comics interesting and sucessful - or rather, giving it lip service but not following through - seems like a bad idea.
This happens to tie into my Comic Book Pet Theory #2 (#1 is the thing where I say that Watchmen is all about gravity and that Dave Gibbons doesn't get enough credit for it): apart from a few early homages to The Spirit in Daredevil, Frank Miller's work bears no relationship to anything Will Eisner has ever done. Miller keeps trying to pass himself off as an "Eisner acolyte", but they are completely different artists. Eisner is interested in the city - in its geography and in the way that geography shapes the lives of its denizens. It's the line that runs through his work from The Spirit to the graphic novels. His heirs are Ben Katchor and Chris Ware. Miller is an action/adventure cartoonist working with outsized figure-drawing and Steranko-inspired page layouts. Though I don't think he's ever said this, he's a follower of Kirby, and, to the extent that you can talk about his innovations, the main one is that he either (a) puts exaggerated Kirby-style super-hero figures into non-super hero genres (Sin City, 300) or (b) brings various noir elements to bear on super-hero comics (The Dark Knight Returns, Batman: Year One). (I think his best super-hero work is The Dark Knight Strikes Again partly because he's dealing with super-hero comics on their own terms, even if he seems to find those terms somewhat retarded).
It's (unfortunately) not surprising then, that a cartoonist whose work centers on figure-drawing would make a movie that is so (detrimentally) "figure"-centric (and I don't say actor-centric because none of the people on screen are creating characters or playing objectives or doing any of the other things we like to see actors do). That's partly why Central City is such a non-entity: Miller is only interested in it as a backdrop for highly-stylized images of movie stars and would-be movie stars. The problem, as Michael Barrier would point out, is that, the drawn figure tends to be much more expressive and much more congenial to exaggerated expression than bodies on film. So Miller is trying to do something he knows, but he's working in a medium where it's very hard to do that kind of thing and he's not using the right kinds of tools.
I'd still argue that what he's trying to do, doesn't really make a good fit with the material, although that becomes a side point as Miller doesn't even do what he seems to be trying to do all that well.