How many times has knowing what you were getting into with a movie kind of... lessened the impact? Predator is pretty much advertised from the start as men vs monster, but what if it had been advertised like it was Commando? Nothing in the first thirty minutes really betrays what you're getting into later in the film. Imagine the shock when you find out the main villain isn't guerrillas, but an invisible alien hunting the heroes down.
So what movies do you all think would have been improved by ignorance of what you were getting into?