Simply put, one of the scariest movies I've seen. Scott (whose subsequent film, BLADE RUNNER, just missed the cut), making only his second feature, cannily fused the horror and sci-fi genres to create a creepy haunted-house movie in outer space. Sure, everyone now knows Weaver's Ripley character as a tough-talking hardass, but she's one of the crew here, and it's interesting to see her as she earns her action hero stripes in a trial by fire (or, more appropriately, a trial by acid-blood). Likewise, everybody knows about the chest-bursting scene- sorry if I spoiled it if you didn't- but what makes the film frightening even to people like me who've seen it dozens of times is the atmosphere of dread set by the film's chilly, sterile visual style.
See also: Blog entry at Film Dribble