Asana
Celebration creatures make completing tasks a little delightful.
Asana, one of the most widely used project management tools, keeps its gamification almost invisible: an optional setting makes celebration creatures (yes, including a unicorn) fly across the screen when you complete tasks.
It is a microinteraction rather than a system: no points, no levels, just a moment of delight at the exact instant of accomplishment, and a setting to turn it off for those who want none of it.
The design lesson: in serious work tools, celebration can be the whole gamification strategy. Reward the moment of completion emotionally instead of numerically, and always give users the off switch.
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