“What’s your favorite behavioral incentive?”
If I understand this right, I’d say how in Burning Wheel, in order to advance skills you need a certain number of tests of certain difficulties, and it doesn’t matter wether you succeed or fail.
So, you’re encouraged to take on tasks beyond your ability, because you’re not going to get any better otherwise, and since they are beyond your ability, you’re most likely going to fail, and failure is fun. Failure drives play forward.
I love this both from a design POV and just a human POV. If you’ve ever genuinely practiced a skill, you now that attempting things you know you can’t do yet is part of the process. You stagnate otherwise.