Ok, so this is going to polarise you guys a little, but MY number one mistake in gamification design, as I personally see it…
Designing the solution that you want rather than what the client needs. Note I am saying what the client needs not what they want.
When designing a solution, it is easy to get caught up in how cool and how fun it can be. Then you start to work on great designs that you love. You take them to the client and you start to explain why your idea and design is absolutely the best solution. Soon you are building your personal pet project, rather than actually solving the problems the client had.