Game designers must be studying Half-Life 2 closely, trying to figure out why it works so well. It's especially puzzling why a non-FPS fan can enjoy it so much. It's easy to throw out the usual reasons: good gameplay, immersive environments, incredible graphics and physics engine. But none of these really explain how developer Valve has managed to once again create a game where everything just clicks. Maybe it's like some movies: you can cite the screenplay, acting, and cinematography, but sometimes there's a certain chemistry that comes together that no director in the world could have planned.

Comparing Half-Life 2 to another game may offer up some of its secrets. The one it most resembles is Call of Duty (team work, scripted events, detailed environments), but it's Doom 3 with which it's most likely to be compared, more because of each game's stature than their similarities. The fact that Valve stole id Software's crown in 1998 and held on to it in 2004 probably has something to do with it as well.

But Doom 3 makes for a good comparison because it tried to accomplish what Half-Life 2 did and failed. The differences between the two games are telling.

Doom 3 relied too heavily on monsters jumping out at the player. It's the gaming equivalent of having a cat suddenly leap into a movie frame accompanied by jarring music. No matter how successful this technique is, it's a short term and easy effect. Half-Life 2 relies more on building tension, often using sound effects to indicate the enemy is approaching.

For mood, Doom 3 went for the obvious: lots of shadows and darkness. (In fact, the game is too dark). Half-Life 2 goes for the more subtle yet far more eerie technique of having many scenes occur at dusk. The orange tinge that hits the top of buildings and blankets the hills adds a melancholy tone to the game.

Doom 3 has the player join the rough and tumble world of the Marines, thereby playing off of an all-too-obvious power fantasy. In Half-Life 2, the player becomes Gordon Freeman, a scientist complete with nerdy glasses, whose allies include other (sometimes older) scientists. (Check out the photo on the wall of Dr Kleiner's office; it features their group...but with villain Dr Breen's face erased.) He's a much more enduring and original choice for an action hero.

With all the darkness and grim Marine demeanors, Doom 3's serious tone couldn't help but become a little pretentious. Half-Life 2, while rarely letting up its atmosphere of dread, manages to have lightness about it. It includes moments of humour, but never devolves into camp.