Torpid what you describe as "looking good" is simply textures. Sure Doom had a lot of steel textures and sure Quake1-3 had a lot of brown textures. But that doesn't define how well a graphics engine is. Anyone can sit down and make a completely pink map for Doom3. It just so happens that Idsoftware wanted to make an environment that LOOKED like a mars base.
It is also much harder to make your game look good with a limited color palette (think about it). But you are right that it is easier to make a game look good with a limited variety of environments. Although the question is what do you consider an environment. You have to realize the complexity of the Doom3 levels that were presented to you. Go back and look at some of the generators, the bridges, the deformable objects. Once you realize how COMPLEX these levels were, you will stop thinking it was so easy to make it look good. You have to remember that every pipe has to mapped and created by someone. It is much easier to make the outside of a building and only show a few windows than it is to generate the entire guts of a building.
It is also much harder to make your game look good with a limited color palette (think about it). But you are right that it is easier to make a game look good with a limited variety of environments. Although the question is what do you consider an environment. You have to realize the complexity of the Doom3 levels that were presented to you. Go back and look at some of the generators, the bridges, the deformable objects. Once you realize how COMPLEX these levels were, you will stop thinking it was so easy to make it look good. You have to remember that every pipe has to mapped and created by someone. It is much easier to make the outside of a building and only show a few windows than it is to generate the entire guts of a building.