With Ridley Scott's excellent direction, Giger's depraved yet elegant design and the dirty, less is more aesthetic, the film had real atmosphere and real terror. The genius of not introducing the Alien for such a long time allows the crew to come across as a real crew, as relatable and ordinary people. You attach yourself to each of the crew members long before anything goes wrong. The superb acting of Sigourney Weaver, John Hurt, Ian Holm and the rest of the crew is near flawless and it adds serious credibility to the events of the film.
Then there is the Alien itself. It's elegant and sleek and, above all, terrifying. The beast is supposed to be the perfect organism and I think it's longevity in film and in other media is proof that it is. Then you have the more mysterious elements like the Space Jockey, something which has inspired decades of discussion and which, hopefully, the Prometheus films might eventually explain.
Ultimately the relatability, terror and mystery of the film make it seem as amazing today as it was back then. It's timeless, scary and can be taken seriously. Something a lot of horror films can only dream of.