Success, at the end of the day, has everything to do with an experience’s impact on its audience. The work you put into the system, its functional qualities and its ability to scale help determine efficiency, while emotional outcomes (was it pleasurable, motivational, immersive?) drive engagement, return use, and retention.
You can enhance an experience by introducing key concepts (beyond user levels and badges) like visualizations and mental models that show you how to improve through interaction. Every product or experience should have a tangible, positive impact on someone’s life - game design principles show us how to ensure people feel those effects.