At the MI6 marketing conference earlier this week, various Microsoft staffers presented a talk outlining how they successfully marketed Halo 3 to the masses. They explained how their main goal in expanding the audience for the third instalment of Bungie's saga was to put a more human face on Master Chief.
"The marketing statement was really based around Master Chief. We did some research, and found out that people thought of Master Chief the same way they think of Robocop and the Terminator, a killing machine... we had to humanize him to reach a broader audience."
To this end they created the 'A Starry Night' advertisement, which gave us a rather heart-wrenching look at MC as a child, as well as the famous diorama commercials, which showcased his heroism. They talk also covered the Crackdown beta, which invested players in the game. Hit up Gamasutra for a full writeup of the talk. You'd have thought all the marketing strategy they needed was calling the game Halo 3.