The number one mistake that people make is thinking of gamification and product design as two completely separate processes. If you spend a lot of time and effort designing a product to do a specific thing, and build a good user experience around that thing, adding gamification at the end will only disrupt that user experience.
Interrupting the user flow with interstitials that are not well explained and seem disjointed or tacked on is a surefire way to confuse users and stop them from doing what they're on your app to do in the first place.
If you find that users don't like doing what your app does, your problem is one that UX research needs to solve, not gamification.
The simplest expression of gamification, and the one people usually think of first, is also the least useful. Of course I'm referring to badges, points, and leaderboards.
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