Remember dismemberment? Oh, how we used to adore dismemberment. What we all really wanted from gaming, we thought, was horrifically visible mortal wounding. Ooh, he's got an arm off, and so forth. We're beyond that kind of stuff now.
Which is a shame, as dismemberment rocks. Specifically, it rocks when it's got an actual in-game purpose beyond sadistic fun-time. Rebellion's 1999 sleeper hit FPS Aliens versus Predator is still the best Aliens game ever made, and a lot of that's because of its limb-removing credentials.
Shoot an Alien directly in the head and, with a suitably melon-squelching noise, it'll drop dead. Shoot it in the arm, and you have yourself an amputee Alien. A very angry amputee Alien, spraying deadly acid from its chitinous stump, which eats through your armour and flesh.
Shoot its legs instead and it'll slump to the ground. Then it'll start dragging itself towards you by its front claws, like a cat run over by a milk float, hissing and spitting, and spurting more of that damnable acid from its mangled torso. It's wounded and close to death, but it's somehow more deadly than a healthy xenomorph. It's down on the floor, where you're not used to aiming, able to get right up against your feet and circle-scuttle around you at distressing speed, causing panic to set in.
It only takes one shot to kill it, but then it only takes one shot to kill a piranha, and it'd be pretty tricky to get a bead on that if it was busy nibbling at your toes.
So, dismemberment used as a device for horror and tension, not merely gratuitously. Eight years on, and no-one's picked up on that. AvP might well have Predators in it (and well-realised Predators at that) but playing as one of those hulking interstellar hunters lacks the primal appeal of Man versus Xenomorph.
Even playing as the Alien, still a disorientatingly impressive experience of fish-eye vision, wall-crawling and brain-munching, doesn't feel quite right. If you get
into the Alien's head, you understand it. If you're fighting it, you don't understand it, and that's what keeps it creepy, even with AvP's cuboid old graphics engine.
Rebellion's last great game was Aliens' last great moment. No new film could ever capture the thrill of Alien and Aliens, because we've seen exactly what these monsters can do, time and again. As Ridley Scott once observed when talking about prospective Aliens sequels, "You've got to change the beast." But it's a different matter entirely when you're facing the beast.
Scuttling along walls and ceilings, blessed with perfect vision in perfect dark (while you're stuck with the all-green blur of night goggles), running at speeds you can't even turn at, indefatigable even when half its body is destroyed, AvP's Aliens are inescapably alien. Other games' extraterrestrials and mutants lean towards the conventional - they're either humanoids with funny faces or they're fliers, sitting ducks of the sky. They don't pervert physics as AvP's do.
These aliens are also palpably animal, which is why they have that terrifying persistence. The vast majority of FPS monsters are either some form of intelligent life, or behave in a human-like fashion even if they are supposed to be mindless brutes. The original Half-Life's Houndeyes and Far Cry's ape-like jumpy-guys are two exceptions, but they hardly had that sense of a ravenous, ungodly thing trying to literally get in your face in the dark.
That's why playing as the Marine, which on paper seems to be defaulting to a stereotypical FPS experience, is the smart thing to do in AvP. The blip of the motion tracker, the trustiness of the Pulse Rifle, it's like genetic memory, an experience utterly familiar and all the more effective for it.