The TF2 "theme" is far more psychological than it is visual. However, the visuals are what communicate these ideas.
Here's some word/phrase association to illustrate the idealogical differences between the two teams:
RED
Working man
Jury-rig
Tradition
Old fashioned
If it ain't broke, don't fix it
Labor
Camaraderie
Nature
Humanity
Family
BLUE
Power
Organization
Technology
Progress
Strength in numbers
Order
Structure
Industry
Collective
Business
Warm colors and angular architecture is not what defines the Red style, they're a result of the Red ideals. The same applies to blue's cool colors and orthogonal architecture. The types of props and locations created support these ideas.
In Valve's "Illustrative Rendering in Team Fortress 2" document, they discuss world composition and texture creation. More than once visual noise is mentioned as being something to avoid.
"Furthermore, we believe that high frequency geometric and texture detail found in photorealistic games can often overpower the ability of designers to compose game environments and emphasize gameplay features visually using intentional design choices such as changes in color value."
Level designers should always be aware of the scene they're designing. Lighting and color help direct the player's attention to where it needs to go. In single player games like Half Life 2, high levels of detail and realism can work because a player can take their time to look at their surroundings and make decisions. A player is also not likely to visit a location more than once or twice. TF2 is much more fast paced. Players need to make decisions quickly, and they are likely to be going over the same areas over and over.
On a surface with a lot of visual noise, a player is more likely to ignore any additional noise. On a clean surface, a player is definitely going to notice if it's pocked with bullet holes or covered in blood splatters, and realize that someone has built a sentry around the corner since last time he came this way. On a highly realistic red brick wall, who is going to notice those red stickies as they rush by?
Impressionistic visuals are as the name implies. The viewer gets the impression of what something is supposed to be. That's all a player of TF2 needs. The player gets the idea of wood, and moves on to pay attention to more important things. It simplifies the information a player needs to process. It makes it easier for level designers to compose a scene that stresses pathways and defensive positions.
Another very important factor of the impressionistic, illustrative visuals though is that of the world it conveys. TF2 is very much a world of fiction. It is a world of pulp, of B-movies, of propoganda posters. It is an exaggeration of reality. We see this in the materials and architecture, but we also see it in the character designs and HUD elements. We see it in the font. We see it in the weapon models and engineer structures.
Youme is dead on about consistency being the most important thing, because otherwise you're just confusing the player by sending mixed messages. Maybe you want to have a different psychological theme. Maybe you want to convey a sense of gritty realism, where players feel the danger around the corner is so real when they jump out with guns blazing their heart is pounding in fear. But to do that, you would need to change everything else mentioned above, not just the map. This is why Youme suggests finding a better game, where you don't have to change the entirety of the game beyond the map in order for the psychology of the art to mesh.
I'll put this bluntly. A person who creates art without at all considering the psychological impact of their art has already failed, unless the exercise itself was the goal and the piece is never meant for any audience beyond the artist.
So again, people should think about the game they are creating a map for, and whether or not the artistic goal of their map matches the artistic goal of the game. If you want to create a photorealistic map or a saturday-morning-cartoon map or a cyberpunk map for TF2, you are not just selling short the game, you are selling short your map. Why would you not seek a game more appropriate to the theme you are trying to create?
