Doom 3 isn't scary. It's slow paced and environments are bland. But there's something about it that makes it great.
Doom 3 plays like Doom 1 in 2 only in that there are demons, and you must shoot them. While Doom 3 has this mechanic, it is much slower paced. Often times the PDA and other things break up the action and slow it down. The environments don't help the pacing, in fact, they make it slower by being drab and grey. In fact, theres a keyword there: Grey. Grey is an ugly colour, yet EVERYTHING in Doom 3 is Grey. I remember when Doom 3 came out everyone was 'OMFG AMAZING GRAPHICS!' and while the environments looked nice and the animation and shadowing was great, the lack of COLOUR killed the graphics. Get used to gray, silver, blue and red. They are the only colours in this crayon box. Theres another small gripe. Obviously, they can do red, but why are even THE CACODEMONS GREY!!? Cacodemons were Infamous for being big, floating RED balls.
Okay, so you are probably thinking: "Why did this gal give Doom 3 a good rating when she's mostly been whining?!" The reason why... Doom 3 will immerse you. The atmosphere is amazing. Not in the sense of 'scary'. Doom 3 is not scary, its scares are dull and predictable. The only scary moments are with some of the ghosts that inhabit Mars City, especially the 'They took my baby' ghost. The thing that makes the atmosphere great is how MUCH detail was put into this Sci-fi universe. There are moments where you just stand, dumbstruck at the thing happening before you where you go 'Wow.' Despite bland colour palettes and walls, whenever you find a new piece of technology in the UAC base, you HAVE to investigate it. They are so interesting, and thoughtful. This is one of the things that makes Doom 3 great.
Another thing, is just that even 15 years after the original doom and 4 years after Doom 3, Blasting demons is still good ol' fashioned fun.