Doom3 is incredibly well done. I always praise Half-Life because it creates such an immersive, realistic experience. Half-Life is enjoyably creepy, but Doom3 is truly scary. The basic foundation on which they build the atmosphere is the graphics - it looks realistic enough to allow for suspension of disbelief; you can really get into it and forget that you are playing a game. It's the first game I've played where everything is rendered with the same detail, in contrast to older games that used lower resolution 2D textures for less important things that you can't interact with, like walls and chairs and desks. In those older games, anything you were meant to notice and use stood out sharply from the rest of the environment because it looked more "real" - it was rendered with more detail, usually as an actual 3D model rather than just using a texture on a flat surface. In Doom3, EVERYTHING looks real. Exploring the Doom3 world is a lot like it would be in real life - when you see a counter with a bunch of papers, soda cans, etc scattered across it, you have to go look and see if anything there is of use to you.

Also, the Doom3 environment gives the impression that it's not purpose-built. In pretty much every other FPS I've played, Half-Life included, if you come across an unlocked room or hallway or ventilation shaft, it's there for a reason - maybe it's not of use to you now, but at some point in the game, you will need to go through there. In real life, of course, buildings contain tons of rooms, few of which you might need to access in your typical escape from invading demonic minions from Hell. The usual game-maker's solution is to simply make any door the player doesn't need to go through locked, and any vents the player doesn't nead to break unbreakable. Modeling the environment in 3D FPSs takes a lot of effort, and it's certainly of little value to go to the effort of modeling a room that the player will never really need to do anything in or with.

But from the little bit that I've gotten to play Doom3 so far, it seems that they've gone through the trouble of doing just that. There's hallways that lead to rooms with nothing of use in them, just random scientists or maintenance workers going about their business -- well, until the "anomaly" occurs and humans start becoming transformed into zombies, that is (hope I didn't just spoil the game for you). Of course, I haven't gotten the chance to play it all that much yet, so I suppose it's possible that sometime later in the game those rooms, etc. will become important.

The other thing I like is that each person has an identity. In Half-Life, all of the scientists were one of these guys:

In Doom3, on the other hand, everyone has a name and a job position and looks different. And as far as I can tell, this serves no purpose other than enriching the game experience. Everyone you met and talked to on your way into the base becomes just one more zombie to shoot and forget on your way out. I think that's really damn cool.

Once the realism gets you immersed in the game, they certainly do a great job of making your new world frightening. You find yourself pulling out your flashlight to investigate dark, shadowy corners where you can imagine all kinds of things lurking. Walk into a control room and a zombie in an adjascent room throws himself at the glass trying to get to you, causing you to fire off a shot in startled panic, causing the computer you happened to be pointing at to send sparks flying. And all that realism makes for a very freaky experience when playing the game at 5am with the lights off and the door locked :)