I Principally, my work has been with a variety of proprietary level editors. I've used some off-the-shelf solutions like Torque 3D, Unreal and Maya 8.5.
My emphasis is on "real spaces." I do my best to create levels that feel like real places and still maintain the necessary directed experience. I make every blocked avenue justified and open space feel like it belongs in the game world. Naturally the experience of the level takes precedence over being a "realistic" space. Level design is like stagecraft, needing to look as much like a real-world location as it needs to in order to provide the player in suspending his or her disbelief.
"I always like your levels. They're a dream to work on." - Level artist I worked with.
Clear communication is also important in level creation. Not only do I want my levels to be clearly understood by the player, but also the level artists.