The first time I saw this, I was underwhelmed. It's many people's favourite of the series, but it's largely the art direction that makes this remembered at all. It looks amazing. H.R.Giger really earned his Oscar and Ridley Scott did well too. But as a story, it's as simplistic as they come. See an alien, get killed by an alien. Once the big scary mother's up and running, 'tis the same formula you'll see in any shite slasher movie (Freddy, Jason, whoever). The cast gets bumped off one by one until the heroine undresses for one final confrontation with the bad guy.
After another viewing, I now think I was a little over-critical.
Perhaps it's a simplistic yarn, but that's horror for you. At root, it's a simple genre. Horror stories will frequently stop dead for scenes that do nothing but try to scare you shitless - which is why some people just don't get them. Alien does everything so right that you could hardly improve on it. It creates an utterly convincing environment for its truckers in space, bitching about their bonuses and being screwed by the Company. You're never drawn out of the story by a Star Trek moment. The Nostromo's crew might not be particularly nice people, but they're all too credible and brought alive by a damn fine cast. Yaphet Kotto is way cool and was that really Ian Holm? Woohoo!
The Alien has attracted much praise (and rightly so), but the spaceship sets are arguably even more important to the movie's atmosphere. Ridley Scott creates a kind of squalid beauty, finding poetry in cooling stacks, computer banks and that condensing chamber where water patters down non-stop. It's almost hypnotic. Had this been filmed on a TV budget with BBC cardboard corridors, it might struggled to compete with an average episode of Doctor Who... but it wasn't. This film took the SF genre, then associated predominantly with the plastic-fantastic likes of Star Wars, and dragged it into a grimier, more realistic world for the sake of the horror. Fear is always strongest when it's close to home.
Continuity with later series entries is a little odd, though you can't blame that on this movie. The Alien is indestructible according to Ash, who might be lying but we certainly don't see it disintegrate or turn crispy even when blasted by spaceship engines at the end. Ash himself seems to know rather more than the Company guys in Aliens, especially if you're really paying attention to Ian Holm's performance. For example, I think he's lying when he explains that his motion detector works on "microchanges in air density". "My ass," adds Sigourney as a coda to this bit of technobabble shortly afterwards. He knows how to detect the alien and he's simply handwaving to cover this foreknowledge. There's also that famous deleted scene (included on the DVD) in which we learn exactly what the Alien did with Dallas, which would have contradicted pretty much the entirety of Aliens.
Mind you, I disagree with the fan theory that the Alien shapeshifted into Jones the cat and got carried by Sigourney to safety when the Nostromo blew up. Admittedly it's already grown like a mad bastard from being that little thing inside John Hurt's chest, so shrinking again isn't so unthinkable? Obviously the theory falls down if you take the sequels into account, but more importantly at the end Ripley is stroking Jonesy. Her last words aren't "Ripley, last survivor of the Nostromo, signing off," but "Come on, cat."
There are a couple of themes which would be continued in the sequels. Capitalism gets a hammering, not personalised as with Burke in Aliens but instead dominating the lives of every single cast member. They're the Company's people, always bickering over pay and bonuses and considered merely an expendable commodity by their employers. The dehumanisation of the worker could be seen as taken to its ultimate extreme in Ash. Even the spaceship's name, the Nostromo, is a reference to Conrad's Heart of Darkness, which was all about the 19th century's rape of Africa for financial gain. Meanwhile the film is also addressing sexuality and motherhood, with the crew being a dysfunctional family under "Mother" while the monster is a rapist that makes you give birth in a bloodily horrible way.
If you want exciting sci-fi, look elsewhere. Alien's style is too realistic for fantasy, its pace is too slow and its science is dodgy. How does the Alien grow from cat-size to man-size in a few hours, eh? However as a horror film, it's top-notch. It has some screaming tension and John Hurt's chestburster is still a shocker even after three sequels so far and a comedy cameo in Spaceballs. Rightly famous.