The thing about fans is that they've already seen the movie in their head years before anyone greenlit the actual film. I'd like to think most fans of a given thing know that the movie in their head won't be the movie they see on the screen, but there are many who won't accept any deviation.
Some of the most successful adaptations so far, though, are the ones that have made the fewest changes to the core formula. I think that's what we get hung up on. We can accept trimming out a key character here, or a dubious casting decision there (Remember when they announced Ledger as the Joker?) but if it doesn't FEEL like the thing it's adapting (I'm looking your way, Transformers, Fantastic Four, Electra, Catwoman...) then it falls apart.