On Xbox One, games are just games. Whether it's a downloadable retail game or what would formally be known as a Live Arcade game, they're all the same in the Xbox Live Game Store. One consequence of that decision is that there is no demo requirement for games on Xbox One.
"Not all games will have demos like Xbox Live arcade games have had on Xbox 360," Microsoft's Marc Whitten said.
Microsoft's new policy matches the one PlayStation has had for the entirety of PS3. That decision has sometimes been criticized, as games would less frequently offer demos on PlayStation Network. "That said, we are going to work on lots of ways to make it easy for you to find and try new games on the service," he told IGN (via OXM).
Retail games will also be offered digitally on Xbox One. However, unlike Steam, games cannot be preloaded before release. "Not at launch, but you'll see us do this and much more over the life of the program," Whitten pointed out. You will be able to play games as they download, however.