Add elements of scarcity and impatience to your product.
Scarcity and impatience are dark hat gamification strategies, but used correctly, they can be very powerful. You can observe that these elements are used in almost all popular products. Facebook was limited to only Harvard students at the beginning, Twitter has a limit of 280 characters, and many modern mobile games can be played for only 30 minutes before you have to wait for more lives.
Simply stated, if a user can do everything without limitation, they will become bored very quickly. Adding more limits will add more fun.