So when you first boot up the game, we’re going to ask you to log in,” he explained. “And when you log in you’re going to get the Drivatars and you’re also going to get a whole bunch of content: tracks and cars. Our production schedule is such that we are putting them in as late as possible and that means making them free as downloadable content on Day One.
We’re not making a launch game. We’re making Forza 5, at launch.
“[But] that is required content to play the game. We basically have designed the game to work with all that content no matter how late is coming in, in order to make the biggest game possible.”
In other words, because games have to be submitted to Microsoft testing, certified, and then pressed onto discs and shipped, Forza 5 has to be done much, much sooner than November. By requiring part of the game as a download on launch day, it gives Turn 10 extra time to finish everything. And so what you get on the disc you buy at the store won’t be the entire game. You’ll need to download the rest of it from Xbox Live (which should be possible to occur as you play, Greenawalt clarified).
I actually kind of see their point. It really must be a pain in the ass task getting a launch title ready and making sure it's a good game with a lot of content, but with that said...