As a PC gamer doing an "outside looking in" thing here, I really wonder how they will handle the open nature of this game on a closed system like the Xbox 360. It will really be interesting to see how it works out.
Probably like Team Fortress 2 on consoles. A stable, static game with little or no updates or extra content over time. How Microsoft has been opening up the Kinect hardware could mean that they're planning on making the Xbox a slightly more open platform, which could mean the possiblity for mods and steadier updates though.