The cast: Sigourney, Yaphet Kotto, Ian Holm, John Hurt, Harry Dean Stanton and Victoria something-or-other (sorry!) are all brilliant.

Plot structure: For the first half of the movie, there is no protagonist. How cool is that? The camera moves from one person to another, framing them in relation to each other and to their ship and to key concepts in the film. It is only as the tension builds that it becomes evident who is best suited to lead, and it is purely because of her abilities and personality: she doesn't freeze in a panic, she is able to think clearly and resourcefully in a crisis situation, she is intelligent about risk and safety. She is not singled out for her leadership role through anything physical, or due to the structure of hierarchy on the ship (she isn't a captain, or an expert on anything). It simply becomes evident that she is a survivor because of her character. Go Ripley!