@wkempf: A pad is more or less mandatory for PC gamer nowadays. Many games on PC are much more playable with the pad since they are ports from consoles. Some games are even unplayable (Dark Souls) without the pad.
Steam Box gamer's has more choice because they can use mouse+keyboard. Mouse+keyboard makes FPS, strategy (Total War, Civilization, ...), simulators, MUDs, older games, etc. available for gamers additional to more casual pad/console -games.
Steam has 50M users.