According to the developers at Turn 10, Forza 5 will be unplayable without a mandatory day-one download. After installing Forza 5 on the Xbox One hard drive, players will have to start a download for the game because the game will not be in a complete state after the installation. The developers say it's due to content being added late in the game's production schedule.
Dan Greenawalt, the studio head at Turn 10 Studios, said in an interview with IGN, "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 new content: tracks and cars. That is required content to play the game."
So unless you have an internet connection, you won't be able to play Forza 5. Keep that in mind when you're purchasing the game.
Another feature that will require internet connection is the "Driveatars" — the ghost AI drivers. The only way that people could train them is if they're connected online. Greenawalt said, "Rather than having just a launch-day set that was created by us, ever day that people race is going to make the Driveatar set that much more accurate, that much more diverse, that much more interesting."
It would've been better if there was a way to play the game without a required patch at launch. It's just seems weird selling a boxed game that you technically can't play.