The fourth act was unnecessary (spoilers)
By fourth act I mean taking John Cusack back to the room after he had apparently escaped the room earlier. I didn't like that the movie went back to the room because, frankly, everything that took place inside the room was rather boring.
The first act was good as a gradual build up, especially Samuel Jackson. The second act with Cusack in the room was typical horror stuff, which offered nothing original. But it was necessary as a prequel to the third act, when he wakes up on the beach and everything turns out to be a dream. The movie should have stopped with that. They could have added some additional stuff, possibly Cusack having nightmares about the room, but there was no need to take him back into the room. That messed the movie up completely.