From stasis cages and poorly lit tunnels to the deep isolation of space, and considering how small the sets appear to be, Alien is vividly claustrophobic. It succeeds both on the horror and science-fiction levels. It's disorienting from the start and confinement isn't even the horror of it all. There is a giant extra-terrestrial aboard the ship and it's more a monster than a cute humanoid.
The beast is gradually revealed but never fully. Mystery and build-up are some of the many strengths of the well-paced script. There is unifying rigor in the creature and ship design. The rooms aren't just atmospheric; they are conveniently built, from the storyboard phase, to inspire distress. In a way, after all, this is a slasher taking place in space with, for victims, bored public workers.
The cinematography is a delight; always mastered, always vibrant. The effects are something else. If you needed a reason to fear alien invasion, this is it. They are depicted as smart but too savage, too animalistic to negotiate. Dense in detail and scientific procedural, Alien is high caliber sci-fi that's virtually flawless on all aspects and speaks to a rather intellectual niche.