This was arguably due to keeping very close to the themes of the movies, something the AvP movies should try sometime. Remember in Aliens, when the marine's radar screen went nearly white with all the advancing aliens? Check. Remember the later scene, when it's implied one alien could take on the whole squad? On default settings, aliens can casually walk through a roomful of marines, limbs slashing wildly without even trying to target, and look back and see no movement. And then there's first-person skull-chomping mode...
The Marine campaign.
Let's set the scene. You are, of course, the only human in the area. You can see the bloodstains where other humans were, but at the moment you're alone. Most of the lights aren't working. You don't have a flashlight. You do have an infinte supply of signal flares, but only three can be active at a time, they don't last very long, and they don't really help, what with the flickering light and the sputtering hiss they make. There is no background music, no cues to listen for besides the sound of claws on metal. And there is, always, your helpful Motion Tracker:
And then, faintly, over the mechanical racket, you hear the claws. And the snarls. And the squeals. And you can't see where they're coming from, and by the time they show up on the tracker they've already moved, but you know they're close. But on the bright side, after playing this game, everything else seems a little less scary.
The fact that nothing happens happens throughout the first mission did not help. Besides the predator killing some of your fellow marines, also freaky. The occasional bleep, the goddamn cat scares (including the above-mentioned), 'Nothing Is Scarier'. The worst part is that at certain points, you can hear the bugs moving around.
On the subject of the second game, there's this lovely part where you play as a chestburster graphically chewing open a man's viscera and ribs. If you really want to see it, you can find it right here. Sleep well.
Not to mention the generous helping of Fridge Horror when you play the Alien and Predator campaigns and discover how horribly exposed you were the whole time - to the other two species you basically glow in the dark.
There's also Extinction, which is AVP as an RTS.
In Requiem there is a particularly bad scene involving a pregnant woman and a chestburster.