Over at The Gathering there has been an interview with Jess Cliffe and Mike Dussault from Valve Sofware, quite a nice interview:
"Q: What is important when making a level for a game such as Half-Life2?
A: Prototyping a map with placeholder textures is a good thing to do, because then you can test the gameplay before you invest in making the map look good. Make sure to playtest it a lot. Watch over someone’s shoulder without saying a word, and observe and make notes. Feel free to send us your map, and we will provide feedback."
Give the whole thing a read here