Ambiance is what really sells the game. Sure the game is graphically beyond just about anything available on either the PC or consoles, but it's the details that are what really make the entire experience come to life.
After stepping off the shuttle, you're presented with an atmosphere that can only be described as something plucked from the darkest corners of the Disney Imagineers' minds. In addition to videos proclaiming the glory of the UAC, random sound cues go off all over the base asking people to report to certain areas or depicting arguments between workers. This all helps to sell the experience and really made the game for me. If you thought the opening walk through the Black Mesa facility in Half-Life was impressive, you haven't seen anything yet. Everything just feels right. As the game progresses, the immersion only gets better and helps to push the game's fear factor to a new level. Few words can describe how cool it is to hear garbled evacuation commands and the screams of your comrades through the static-filled radio.
One of the bigger issues I, as well as just about any gamer, had after seeing the first Doom 3 shots two years ago was wondering if my PC would be able to run it. The good news is that it is totally possible. Even on the lowest game settings it looks great. While it might not match the high-res screenshots pixel for pixel, you won't be disappointed.