IGN pumps out more of their GDC coverage by taking a look at Portal, the Companion Cube, and GLaDOS.
Initially they thought that GLaDOS should be a devious boss, as with most games. They initially designed her to use James Bond like lasers. However they came to the conclusion that lasers were bad. In their initial build of the end boss, the gamer didn't really pay attention to GLaDOS, and the original twitch based difficulty really alienated players who enjoyed the more casual puzzle solving game play.You can read more about the design decisions that went into Portal over at IGN.