Style
Part of the appeal of "TF2" comes from its distinct artistic style, featuring larger-than-life characters engaging in over-the-top violence. Design your map so that the architecture and landscaping complies and meshes with this style. Breaking from this style creates a visually unpleasant disconnect between the characters and the environment, and maps that contrast against character and combat aesthetics will drive players away.
Lighting
Ensure that your map has adequate light source distribution and that each area's lighting fits with the area's overall design. Consistent, effective lighting allows players to play your map with maximum efficiency, while inadequate lighting makes navigation and combat difficult. Never leave an area unlit, even if the area's nature makes leaving the area unlit seem natural, such as sewers and tunnels.
Interiors and Exteriors
Create a balance of interior and exterior locations when creating a map, and create natural-looking interconnections between interiors and exteriors. Maps that lean too heavily toward interior or exterior locations are boring and will benefit classes that excel in that environment while diminishing the effectiveness of other classes. Mixtures of interiors and exteriors stimulate the eyes and allow all classes to play at equal efficiency
Feedback
Release beta versions of your maps at regular intervals during the development process. Every player who tests your map can provide feedback on the map's design, pointing out oversights and offering possible avenues of future design. Releasing multiple beta versions will also make it easier to correct problems with your map's design, as addressing issues in well-developed areas will be more difficult and time-consuming than addressing issues in areas that have been created recently.
Performance
Monitor the resource requirements of your map during the development process. Resource-intensive maps may look appealing in a still image, but frame rate reductions will make game play on the map a choppy experience. Split large, resource-intensive areas into smaller areas with walls, hills and barricades. Smaller areas also allow for more intimate combat and allow players to learn the map's layout more easily.