Doom III was greeted with mixed reviews on release and it was practically destined for this because it tried to cater to both fans of the originals (as it should) and to fans of the leaked alpha version (which showed potential as a survival horror title).
Given the art direction, pacing, and play mechanics I would suggest that they should have stuck closer to the survival horror aspect and let the modders focus on the FPS shooting frenzy remakes of Doom 1 and 2.
(In a dream world the folks at Id Software would have stayed with the same shoot-like-crazy titles of yore and we would have a worthy companion to Painkiller or Serious Sam...)
It wasn't a terrible game but it just felt a little off and it stumbled entertainment-wise next to it's (presumed) rival "Half Life 2".
Many, like myself, thought that mods for the game would be it's saving grace and anxiously awaited what the mod community would produce.
The first batch of stuff was disappointing.
We were greeted with the notorious "Flashlight Mod" which supposedly made the game 1000000% better. While it did help keep the blast-fest play style more manageable it killed the remaining spirit of survival horror in the game. Then a barrage of silly decals for the Flashlight followed.
The next wave of mods were slightly less disappointing but still troubling for those looking for new stuff to play. Essentially amateur Shader programmers looked to find ways to enhance the real-time lighting and Normal Mapping effects of the original game further to test their skills with these new capabilities. So we had the likes of better shine, parallax mapping, self-shadow mod, bloom mods. All of these had the effect of simply making the existing game prettier. (Which is not a bad thing per se but does not compare with things like the Ravenholm mod for HL2 that were cropping up around this time...)
Out of this work we did get some ultimate Doom 3 "fixes" like JC Denton's Enhanced Doom 3 (breaths new life and excitement into the game with graphical AND SDK improvements) and supreme graphic enhancement like Sikkmod (test shaders 5, Soft Shadows, SSAO, DOF, HDR) or super texture improvements like the Rygel textures and the Monoxead texture project.
The thing that modders struggled with (and still struggle with) is that the look of the textures changes significantly depending how they are lit and normal mapped. You can't just bake a drawing or photo into a texture as with older engines.
We were fortunate that right from the start some diligent modders kept cracking at the engine and produced arguably some of the best mods ever made.
I humbly submit my personal favorite Mods for Doom III
1) The Dark Mod
Thief 1/2 (classic) style game-play. Not just a mod, a Fan Mission platform with over 30 missions.
2) Ruiner
3) Hexen: Edge of Chaos
The long awaited fan imagining of a next-gen Hexen title.
4) Classic Doom 3
Portions of Doom I re-imagined in Doom 3.
5) Raccoon City Mod
A Resident Evil inspired mod.
6) HardQore 2
Side Scrolling mahem.
7) Doom II for Doom 3
A selection of cherished Doom II levels remade for Doom 3 by various authors
8) Doom II in Doom 3
Benjamin is usually just a generic map machine but this is a fun concept and plays well.
9) Shamblers Castle
Quake re-imagined in Doom 3 (supreme!)
10) Quake II: Lost Marine
(This mod will not run on the 1.3.1 patch of Doom 3 )
(A modern version that works with Quake 4: Quake4.gamefront.com )
The following are Maps but for most folks would also count as mods given their strong art styles and design.
Genuinely scary castle map with occult overtones.
You know that "Aliens" vibe that Doom 3 has? Well this one amplifies that by 1000%
A delicious creepy yet humorous romp in a zombie infested old-timey Tudor architecture town.
Small but fun.
