At the time I was amazed by this movie. The sets are just epic... all practical sets, no greenscreen in '79! It's stacked with name actors, it's dark and foreboding and the last 20 minutes are an absolute pressure cooker. Sigourney inhabits Ripley perfectly, a long haul trucker in space surrounded by a crew of diverse individuals. Some are goofballs, some are consummate professionals and one has a dark secret.
I admit... this is a hard movie to watch now. It inspired so many tropes that it may appear cliché. It's space scenes are among the last practical / scale model scenes in major films, so they barely stand up now. But the story, the characters, the interiors and even the planet surface (a huge closed set depicting a volcanic ash landscape lashed by constant high winds) are magnificent. And that one scene... the shocker... you'll know it when you see it!
Through it all there is a documentary feel to this... the characters talk over each other, they mumble and grumble about their pay-packets, they tell each other to F^@# off. And they all totally fail to be typical heroes. Ripley is not a squash buckling type... she is just trying to survive. This vulnerability is key to the plot.
So... great sets, great cast, a conservative and smart use of practical creature effects, bizarre artwork by H.R Giger and the genesis of so many other lesser movies that nostalgia becomes one of the cast. And there's a cat.