What Makes a Good Map?

Gameplay and level design are what people generally look for in a map. Here's my take on each of these.

Gameplay

1. Atmosphere. Many maps strive for a creepy atmosphere, with somewhat dimly-lit areas and the presence of monsters heard but not seen. Others go for reasonably well-lit areas, and even a brighter look to the map with new blue-sky textures, etc. Each approach works well, depending on the intention of the author.
2. Surprises. Some maps, especially the creepy ones, will spring traps on the unsuspecting player, perhaps without warning. It's best, however, to give the player either a clue that there's a trap, enough health & ammo to deal with the trap, or a way out. The key is to make the player sweat without dooming him/her to failure.
3. Action. Some maps will have enemies sniping at you from a distance, forcing you to duck and hide until you are close enough to deal with them. Others will have hordes of enemies swarming around. Others will require you to run through or retreat or cause enemy in-fights. All approaches can work very well if implemented properly.
4. Health/Armor/Weapons/Ammo Not too much, not too little. As always, keep the player on edge, but give him/her a chance.
5. Skill Levels. Not all players have the same skill. For wider appeal, skill levels should be implemented. Otherwise, in the text file specify which skill level the map was built for.
6. Replay Value. A good map will typically have a high replay value. In other words, people will want to play the level again and again. Among the elements contributing to high replay value are non-linear routes, secret areas that are visible but not easy to access, and varied types of battles.

1. Theme. The most common themes appear to be tech/base, medeival, and hellish. When designing a level, try to stay within the theme, unless the story-line indicates otherwise. That's not to say that some elements of one cannot be introduced into a map with a different theme (e.g., the selective use of MARBFAC textures in tech/base maps). But this ought to be done with some care.
2. Architecture. The better the architecture, the more pleased your audience is likely to be. However, do not sacrifice gameplay for architecture. Most buildings you create will have corridors. Avoid long, straight, and featureless corridors. Make the corridors turn and/or double back; add cross-beams on the ceiling, support pillars along the way, pools of light; skylights; and recessed sections of wall. The same applies to rooms in the building.
3. Map Layout. While there's nothing inherently wrong with a linear map, providing the player alternative paths to a given objective is often appreciated.
4. Map Structure. Avoid a map with the same height attributes throughout. In other words, make some rooms with higher ceilings or lower floors than others, make rooms that are higher up (accessible via stairs, elevators, or teleporters), etc. Add windows, especially to areas that the player will only access later in the map, or secret areas.
5. Secret Areas. Most players consider it fun and challenging to discover some or all secrets. Include secret areas in your maps whenever possible. Provide some sort of clue that a secret exists -- a deliberately misaligned texture, a razor-thin recessed area, a flickering light. Avoid hiding required keys or switches in secret areas, particularly difficult-to-find areas.
6. Texturing. Same principles as Themes, above. In addition, make sure textures are properly aligned.