Probably the greatest contribution Alien made, even more so than Giger's terrifying alien creature, is it's de-glorification of space travel. By presenting the Nostromo crew as working class grunts, the film brings space down to a gritty, more relatable level. And it is that sliver of familiarity that makes the ensuing events all the more terrifying.
I saw an old grainy 70mm print of this projected in Imax and it came off like an Alan Clarke film in space. Fucking brill. The amazing thing is that a high quality dvd transfer makes it look like a very glossy Chris Cunningham music video (or the other way round...), the image just majestically tightens up. Only the look of "mother" has really dated.
You have to give Scott credit for creating a psychologically terrifying sci-fi picture that functions on so many different levels of fear; fear of isolation in space; fear of confinement in a space vessel; fear of deadly infection; fear of a bloody death; fear of the embodiment of horror itself, the xenomorph; and, most importantly, fear of the unknown.