First 5 minutes - some of the best cinematography and direction in the history of film making.
The slow sweeping camera pans across the decks of the Nostromo, with low, sweeping music. The computer sparking to life, the CRT reflected in the helmet.
The slow wake up and building of characters. the atmospheric trek across LV-426, and the first glimpse through the storm of Giger's derelict. The tension and mystery which builds in stunning huge sets, culminating in the attack on John Hurt.
The pacing is sublime, the acting is exemplary across the board, the direction is awe inspiring, the special effects (for a film which is now 30 years old) are (with the exception of the chest burster) still excellent, the climax and fear of being hunted as the film explodes - then the way 'mother' goes against her children in a pseudo-anthropomorphic way is story telling at it's best.
It's what films should be like. A well told tale which you can sink into and let yourself go. It's the antithesis of the wham-bam-no thanks mam mentality the movie industry suffers from today. Formulaic films, identikit story threads, off the shelf SFX - it's all bland.
Alien stood at at the time as a special piece of film, and it stands apart from films today for the exact same reasons. It's as much a moving piece of art as it is a film, but as a film, it's subtle, tense and a joy to watch.
The lack of gore and site of the Alien is deliberate. Ridley Scott understands that fear of the unknown or speculative is much greater than the fear of something present. If you can see what you're up against, you can fight it (or run) - If you have no idea, then it can get you whenever it wants.
I weep for the future of the film industry if people have been so blinded to great cinema by the production line trash hollywood has churned out for the last 10 years since the "CGI Explosion". ILM - I now hate you!