Seraph, why do the Marvel movies do so well? Because people have either forgotten the plots of the comics or learned to get over the fact that the movies weren't steaking to them. It's not sullying a story's reputation to take the premise and make a new plot. That's why fan fiction exists, to explore the parts of a canonical world that weren't explained throughout the official story.
But some games made into movies suffered something known as "pop culturalization." That's why The Rock was in Doom. I like The Rock as an actor, but come on, DOOM!? The Rock should not be in Doom, he should be beating up slave driver in a third world country while backhand his partner every few minutes to remind him who's boss!
Nil, you're right, if they had just used those tactics to get people into the movie, it could have been great. I still found the story interesting, but I wasn't really scared, nor was I grossed out. Not even a full autopsy could gross kids out these days.
Fronken and you others who mentioned Warhammer... Are you guys following the MMO's creation, as well? I only discovered Warhammer through Mythic Entertainment, as I was playing their original MMORPG, Dark Age of Camelot, when they announced the creation of Warhammer. I think it looks great, but I'd love to read the books and play some of the other games (probably not the tabletop -- I couldn't get enough people to play with me) before it comes out, though. But when it comes to the books, I don't know which one to start with.