Doom was a landmark game for many reasons. It was the game that defined the first person shooter genre, introducing (incredible for 1993) 3D graphics to the genre. And really the graphics still look nice today, not advanced, but nice. It had official in-game support for player-made mods. (You can actually find and play levels made by the Columbine school shooters. They're just Doom levels, nothing strange about them at all.) It caused a massive moral uproar when it came out because of it's heavy use of Satanic imagery. The monsters you fight in the game are demons. It spawned a sequel and several expansions, all maintaining the popularity of the first game. Then 2004 came...
Doom 3. No one is safe.
The original Doom and Doom 2 levels were scary because you were never safe. They were intense, and constantly had you on the alert. When something happened you'd often go scurrying across the room away from anything that moved. In Doom3 you're safe plenty of the time... but the beautifully rendered decrepit Mars research facility didn't let you let your guard down. Playing alone in the dark? HA!
But what I find really interesting is that you can tell the difference in tone from the soundtracks of the games- check this out.
It's intense, fast-paced, grim but in keeping with the frantic gun fights of Doom
See the difference? In Doom you fought monsters with powerful weapons, victory may not come easy, but you will be triumphant. You were an armored God of death waist-deep in the blood of the legions of Hell's armies.
Doom 3? You're a scared fucking marine creeping through the dark halls of a demonically possessed research facility trying to find the source of these demons, a search that eventually leads you to the depths of Hell where you face unimaginable horrors. You know from the start that you're fighting a losing battle. At the start of the game when Hell's armies breach the portal to Mars you can hear gunfire and screaming over your radio, but about half an hour later the airwaves turn to dead air with the occasional order over your headset. Eventually even your orders disappear. The only other living people in the facility stop turning up. You are alone. You are doomed.
(In case you can't tell I love Doom 3 more than just about anything.)