Every couple of years, someone threatens to adapt Atlas Shrugged, which I expect couldn’t result in anything other than a spectacular train-wreck.
I read the book after seeing the film and was amazed that Minghella had been able to pull it off so well. It doesn’t exactly follow the conventions of most novels, certainly.
Amoung other reasons is that it is just too damne long. One of the projects that Ms. Rand was working on when she died was a television mini-series version of Atlas Srugged. That is the only way that you could even come close to getting it right.
Also Stand By Me.
I doubt A Confederacy of Dunces would make a good film.
Anything By Flann O’Brien (aka “Ireland’s greatest writer”. James Joyce? Pshaw.)
I know that the screen rights to Generation X were purchased some while ago, but I think he got them back (it was a 7 year option or some such). He said he was just as glad. (Hijack: Coupland came out only a few months ago and has never written a gay main character and since then I’ve een him referred to twice as “gay author Douglas Coupland”.)
David Sedaris sold his film rights to a couple of books and says that as much as he would love to have the money he will never sell them again. The producers made him a nervous wreck wanting to essentially gut and rebuild his work, then changing it from movie to sitcom to miniseries to animation and back again, and when the rights reverted to him (another limited time option) he said he will never sell them again (though he said if his sister Amy ever has the clout to produce then she can have them).
Related conversely:
I really believe that The Time Travelers Wife by Audrey Niffenegger would make one of the greatest love stories ever told in cinematic history. The right studio, the right director, the right actors…definite Academy Award Winner-- Best Film. I’d like to see Charlie Kaufmann write the screenplay!
Illuminatus! by Robert Anton Wilson and Robert Shea. Part of the reason, of course, is that the moment for such movies has passed, but mostly because it’s such a long and sprawling work that jumps around so much, swapping character’s view for character’s view, and loaded with internal dialogue, that I don’t think anyone would be able to pull it off and have it be watchable. (Yeah, I know they did a musical version of it on the stage, but it evidently pretty much sucked hard.)
I would say Vonnegut’s Galapagos would be unfilmable, but they did a pretty good job of Slaughterhouse Five. I’ve yet to see Breakfast of Champions, though.
Anyway, the first novel to come to mind was Portnoy’s Complaint, but I just checked IMDB and damned if they didn’t already do it. Wonder if it’s seen DVD release yet?
I think there should be a law against Hollywood adapting the works of Tom Clancy or Anne Rice. TC’s books get raped by Hollywood every time (yes, I’m even including Hunt for Red October here) and Anne Rice’s are too intricate to translate to film. If The Queen of the Damned had to be made into a movie, it should have been eight hours long, with the prequel (The Vampire Lestat) weighing in at a good six hours.
Only one John Varley novel (Millenium) has ever been made into a movie and it sucked. May God have mercy on anyone who tries to tackle his Gaia trilogy or Steel Beach… I hold out hope that Red Thunder or The Golden Globe could be made into decent movies though.
Any book that relies on first person narrative to present a psychological portrait of a character I think is impossible to translate to film. I always think of Catcher in the Rye as the greatest novel that will never be able to translate to film. Everything interesting about that book takes place in Holden’s head as he narrates his story. The whole movie would have to be one long voice over sequence.
Speaking of Stephen King, I can’t see that Gerald’s Game could ever be made into a movie.
Maybe a horribly violent and gory porno movie. 
Actually, there has been one really good H.P. Lovecraft story put on film. It’s just that the producers of Alien didn’t know it was a Lovecraft story.
Geek Love. I think David Lynch could pull it off. It deals with a family of circus freaks. The older brother, a boy with flippers for arms and legs, ends up developing his own cult and becomes a sort of dictator to his family. It’s very weird, but Lynch did a good job with The Elephant Man.
I’ve actually thought about trying to come up with a way to adapt this to a stage play (why not? I know someone who did Reservoir Dogs as a play).
Dune
The Lord of the Rings
The problem with It is that the final battle involves the hero locking minds with the creature and the battle (with their minds) to push the other off the edge of the universe (or something like that; it’s been a long time.) There’s no way that that can be described on film. So instead they had the kids push a giant spider on its side. Stupid. It made me want to dropkick the TV.
I love The Long Walk, and I don’t know why. If I were to describe it to somebody who hadn’t read it, they’d probably say that it sounded boring. But I’ve read it 2 or 3 times. I think that it would be easy to make into a movie, but hard to do well.
I agree it would be unfilmable (maybe as adult CGI), but I saw an excellent stage production of it in Atlanta last year. It was mostly live action with some assistance from the Atlanta Center for Puppetry Arts.