Gamasutra has a great article about a study that used biometric feedback to measure how much various action games (including Halo 2) engaged players - go see what drives us to play! (Seems like risk plays a pretty big part in how much reward we perceive, and Halo does a decent job at balancing things.) There's a summary of the article here, and the full piece is here.(Louis Wu 16:22:33 UTC)