Doom 3 is a successful followup on the venerable game that was probably most responsible for the popularity of shooters on the PC. Like the original game, it sets a new bar for graphics and design; the textures and lighting are the best we're likely to see from this generation.
The main way that this differs from the old Doom is the way that the experience is planned out. Doom pioneered suspenseful lighting in games, and used ambushes as part of the level design, but there was more of a balance between close combat and large fights in the open. In some ways, Serious Sam is a little more like the successor to Doom with the way that it emphasizes huge chaotic combat with pretty dumb AI. Doom 3 is more dark, claustrophobic, and jumpy. You spend almost the whole game in pitch black, small rooms and corridors, where the only thing you see is the narrow beam of your flashlight, and there's an ambush almost every turn. In fact, this is probably the reason for everybody who voted "1," because they were driven crazy by it.
Doom 3 is very successful if you are looking for a tight, suspenseful game that never really lets you relax. It's so well polished I would give it a perfect score if it didn't get so repetetive after a while. I think they made it that way because of how demanding the graphics are. I would love to see it a little while on, when average computers are more robust, and the level design can be opened out a little for more action like the original.