adding meshes and creating atmosphere is what he prefer. Creating a world, starting with nothing to end with a lovely universe and nice points of view... giving ambiance and a soul to an universe, a world. That is what is awesome in level design.
by giving a great importance to Level design, William search the best ideas in game design to just get some levels awesome and funny to play.
being more powerful than a computer, creating puzzles that stops the gamers for hours. Giving them epicness by killing an hydra, creating maze. Seeing them invest their characters, being them, crying when death is near. that's what happend when he's mastering.