Speaking with IGN Turn10's Dan Greenawalt revealed that Xbox One launch title Forza Motorsport 5 will require a DLC download at launch in order to work.
"When you first boot up the game we're going to ask you to log in, and when you log in you're going to get a whole bunch of content: tracks and cars. That is required content to play the game."
The reason according to Greenawalt is that a lot of content is being finished late on in development and won't make it onto the disc. You could also speculate that this is a result of Microsoft's decision to reverse the online requirement.