The first Alien is incredibly scary. Especially if you walk into it blind.
They spent the first quarter of the movie just establishing the characters. Unless you know Ripley lives it's really a situation where anyone could live or die. There's a lot of creepiness and isolation. Something bad happens to one of the characters then everything's okay. But then, twist, it's really not. They have no weapons, there's nowhere to escape to, and no help is coming. Oh, and the xenomorph is not the only problem.
One of the best things about having children is being able to introduce them to cinema like this. Alien is one of those movies I'm excited to watch with my son in like eight or nine years when it won't traumatize him for life.