For me, a clean, intuitive UI is key. But more interesting, is Streaks.
Apps with Streaks easily strike an emotional cord and become part of my daily routine, e.g., Product Hunt, Duolingo, Fitbit (exercise/sleep), iBooks, etc. I suppose it’s because Streaks give a feeling of progress tracking and consistency.
What about you? What makes an app (especially the non-essential ones) most sticky?
Are there apps you use a lot more than you need to? And can you identify what keeps you engaged with them?