The thief is thwarting the law and everyone is his enemy. He has no loyalties and no one has any loyalties towards him. The thief's life is a life of shadows and duplicity, always remaining unnoticed, whether he's in a busy city street or in a king's castle. All these characteristics are crucial to understand as the developers of a game like Thief. I think Looking Glass understands the thief.
Every element shows a successful effort to make the environment something a thief can actually use.
The sound in Thief is perhaps one of the best uses of sound I've ever seen in a game, with footsteps making degrees of noise based on your speed, and different surfaces such as carpet and stone making more noise than others. The guards and monsters you are avoiding can be pinpointed exactly through Looking Glass's implementation. You can be sitting in the shadows around a corner, but you know there is a guard moving at yea speed in your direction about ten feet away from you. Obviously, lighting is crucial to Thief as well. There are degrees of visibility based on where you are standing in relation to the light, or the lack of it.