The state of being simple or not complicated.
Simpleness is usually seen as a good thing because it means something is easy to understand or do.
Examples:
The simpleness of the design makes it easy to use.
I like the simpleness of the story.
The simpleness of the game makes it fun for everyone.
The simpleness of the recipe makes it perfect for me.
The simpleness of the task made it seem like a game.
I love the simpleness of nature.