First up, the demo will feature 4 player co-op so you can run around the giant sandbox world with 3 of your buddies.
The demo drops June 21st
Finally, and this is new to the demo world, the demo will allow you to unlock achievements and carry them over to the full game.
That version was a generous 30-minute demo that, from what we could gather, will be released publicly at some point before the game's release next month. Interestingly, said demo will let players earn 100 of the game's 1,000 achievement points prior to the game's release, although unlocking all 10 of the achievements will undoubtedly require multiple play-throughs.