Minecraft wiki entry for PC_Gamer_Demo, associated text: If the player is at or above y=89 at time of demo expiration, they can continue to interact with the world.
If the player's game window is at its smallest size at time of playing, upon enlargement of said window, all blocks become transparent until the mouse is moved or until the player interacts with their keyboard. This reveals all underground caves, thus revealing where ores and other valued blocks can be found without the need to dig.
If the player opens two windows of the demo, one of them playing the world, but paused, and the other one, on the home menu, reset the demo world and then click back to game on the other window, an error message occurs, and if the player plays the demo, the time is reset, but not the world, and their inventory items are lost, but not the items in the block entities.