Some critics at the time carped that this was "just" a haunted house movie, with a few jump scares, as if haunted house movies are not worth the price of admission. In fact, it's precisely because it's a high-concept horror that it works so well. ("High concept" means, it seems to me, precisely the opposite of what the words implies, but...oh-well)
But it is a stylish, beautifully designed and shot horror, with an extraordinarily convincing setting that built on the new capabilities of technical filmmaking that was emerging in the mid 70s.
Admittedly, seeing it again now after space and fantasy movies have leapt to new levels of realism, the plastic, foam and fibreglass sets, with puppets, men in monster suits and in-camera effects show their age somewhat. So does the dialogue which sometimes sounds poorly improvised as if for a radio play. ("There's a sort of pit down here, full of leathery objects like eggs...")
But back in 1979, I made the effort to see it on release in 70mm in Leicester Square. The house was packed and we were thrilled by the communal experience (as we were by Star Wars and Close Encounters in 1978 and Jaws in 1975). The lush space suits and vapour-emitting helmets, the grimy interiors of the Nostromo, the gross anatomy of Giger's eggs all combined to convince the audience that we were in space and on a slow but steady roller-coaster that would plunge, pitch and yaw and roll whether we screamed or not. Even the cast felt like a realistic ensemble: no big stars, but character actors with lived-in faces (Stanton, Hurt) and feisty women (Weaver, Cartwright).
Whatever the merits of the later sequels and prequels (or demerits), this was the real deal. Whereas in Prometheus, it was difficult to forgive the basic premise of a multi-billion dollar expedition left in the hands of complete numpties, the more or less complete absence of background story in Alien allowed us to focus on Ripley and her determination to survive.