I keep seeing people ask what makes this game special, so I decided to create a list.
Extremely flexible and animated models. They climb walls, the leap over railings, they walk sideways through door cracks, they come through the floor, they crawl out of vents, they hunch over other corpses, lie against walls, they are the perfectly realistic ragdolls.
Amazing lighting. You see shadows skitter across a semi lit area. Lights flicker wildly. You can see eyes peering out at you in the darkness. You can turn your flashlight on and see a very small portion of your screen and then turn your flashlight off only to be greeted by the darkest environment I've ever seen. You can navigate by the ambient lights of computer screens and alarm systems alone. When you swing your flashlight, you are greeted to a brief moment of complete darkness as the beam from your flashlight shines out behind you, which makes every swing all that much more important. An imp in the darkness summons a fireball and creates a nice orange glow around his hand / parts of his body and surroundings.
Amazing translucency affects (that might not be the right way to say it). Thick unbreakable glass tends to bend the image slightly while thin glass will give a much better view. Smoke and Fire both create a heat haze that distorts everything behind it with amazing realism. A picture on a wall may have a dull reflection of the lights of its environment.
AI. Certain enemies pursue while certain enemies fall back. In most games, if you stand by a doorway, you are pretty much guarenteed that any monsters that 'hate' you will come through that door and into your point blank shots. But this game does some smart things. Enemie marines tend to use obstacles as cover and will do a standard 'unload clip', hide, reload routine. Imps are more likely to rush blindly. The spiders seem to be very good at surrounding you.
The audio is also very nicely done. The voices all seem very realistic and not like someone reading from a piece of paper. The area tvs play awesome company videos that remind me of ethics training and the movie starship troopers. A broken window created a suction noise that was so loud that I couldn't hear anything around me.
The levels also have some very cool designs. A functional maintence bridge that covers overheating containers. When the bridge is retracted by a nearby control panel, a robot will come out and start flipping the containers to keep them cool. And the bridge doesn't retract like your standard FPS, it pieces itself apart into many different 'arms' that all retract differently. Stairs collapse sending you down a level. Doors get crushed, dented, broken. Ceilings break, pipes burst. Giant windows can be sealed via control panel by large metal plates. Ladders deploy from the floor or ceiling instead of already being in place. You can run through heavy equipment, risking life and limb. The 'living' walls are scarily realistic.
I think I am forgetting stuff. But these are the things that make this game different and better than recent things I've played. Some of these details are most likely overlooked because they work so fluidly that you barely notice them happening.