This was arguably due to keeping very close to the themes of the movies. 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 from the same game.
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 have an infinite 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's 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.
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.
Just TRY to play the Marine's campaign at night.
The first game, as noted, was more focused on causing terror in the player, and it shows - near pitch black constantly, none of that cinematic music, more intense gore/death screams, a slower-refreshing motion tracker for the Marines, and immense single-player difficulty, especially as the Predator once you pass the first few levelsnote Granted, the Predator can take more damage and has a medicomp to heal himself while the Marine's health drains so quickly that he's practically a One-Hit Point Wonder, but you couldn't restore your energy manually, and the Aliens just kept respawning on some levels..
At various points in the Jaguar game, you can hear a soft voice murmuring... something, and it's never coming from your character. Just who isspeaking to you?
The Jaguar game in general perfectly replicates the experience of being outnumbered in a crowded space.
The 2010 game:
The facehuggers in the Marine campaign are terrifying to fight. They are small, can run very quickly and in dark settings, you will not see them until they jump at you. While your character can throw them away, they automatically take a third of your health, which makes avoiding the use of stims very difficult.
In a particularly smart move on the developers part, if there is a collectible or ammo you want to get, chances are it has eggs and facehuggers there. If you rush towards the collectible without being careful, be prepared for a Jump Scare.
When the aliens escape. Imagine this if you are a colonist. You think something is up, and then all of a sudden, there are Xenomorphs everywhere. No matter how good you are at hiding, no matter how much you try to escape them, they find you and cocoon you. And the best part, your boss intended for them to get out For Science!.
The marines situation. You know its a big infestation, and then all of a sudden, unknown hostiles blow up your ship, most of the officers are killed, and those same hostiles are killing your own soldiers while the Xenomorphs overrun you.
Karl Bishop Weyland as a whole. While it is not uncommon for Wey Yu executives to be evil, Weyland is far more callous. The guy intentionally sends people who complain about work conditions to the facehuggers, and later lets them out, completely disregarding their lives as he wants to study them. And when the Marines try to save Tequila's life, Weyland spitefully shuts down the surgery equipment. To say he is a sociopath doesn't do it justice.
The Marine campaign on easy can give you scares. On normal, the Xenos are more aggressive, the facehuggers actually try to sneak around you and depriving you of your motion tracker, and also your ammo. You only have a pistol and some shotgun rounds, which a facehugger will dodge.
Specimen Six, while the most sympathetic of the protagonists, is a walking nightmare from the marines' point of view. Here is a Xenomorph who is more intelligent than the average Xeno, who observes your pattern. You hear a Xenomorph screech, and then you get grabbed from behind and headbitten. So the comrades arrive, and there is no sign of the assailant. And that is not going into the fact Six can pull this off in broad daylight.