What makes a good level?
The answer to that question can both fill volumes, and be answered in a single line: A good level is a fun level. Victor Hugo wrote "If a writer wrote merely for his time, I would have to break my pen and throw it away." That is to say, don't design just for the simple sake of designing a level. Rather, design a level to be used. Give the level purpose, motion, and character, and it will be a good level. All aspects of design will contribute to that, and must be balanced against each other. In the end, people are going to judge the level on whether or not they liked playing it.
"A building is alive, like a man. Its integrity is to follow its own truth, its one single theme, and to serve its own single purpose. A man doesn't borrow pieces of his body. A building doesn't borrow hunks of its soul. Its maker gives it the soul, and every wall, window and stairway to express it." - (Roark) Ayn Rand, The Fountainhead
Remember, a good level often uses the characteristics and mechanics of the game in question to their strengths. That is not to say experimentation is key, rather, the aspects of the level should complement the gameplay moment(s), message, story moment, etc. that you as a level designer are trying to get across to the player.
Some Aspects of Design
- Textures help to illustrate the world's art style, whether it be realistic or cartoony. See Material System.
- Models can provide more detailed geometry than brushwork while still creating less of a system drag. See Modeling.
- Particle Systems add effects that are unattainable using textures or sprites. See Particle System.
- Sound adds life to maps, ranging from believable weapon/NPC sound effects to ambient soundscapes that set the mood of your map. See Sound.
- Lighting also sets the mood for maps and can highlight areas that you want to draw attention to. See Lighting.
- NPCs (or Non-Player Characters) are integral to maps. From the Combine to zombies to citizens to Barney, NPCs add the crucial element of interaction to levels, whether that interaction be conflict or companionship. See NPCs.