I'll leave the bulk of my comments until I actually judge things, but Master Chief is THE quintessential video game character of our generation, bar none. I know you and I are pretty much the same age. You can make the argument that the character development isn't there or whatever, but Master Chief is the face of what is probably the most important/influential console* video game series of the past decade. When you say Xbox/Xbox 360, the image that immediately comes to mind for me is Master Chief in full-gear. Halo is the defining video game series for Xbox, and Master Chief is the face of it all. Hell, the series essentially spawned an entire professional gaming league based around it.
And this is coming from someone who's never owned anything in the Halo series and has only played the multiplayer sparingly with friends.
* console being an operative word here....Warcraft, Starcraft, etc. aside.