Peopleware: Teams need a challenge
Good work experiences have always got a fair measure of challenge about them.
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The challenge of the work is important, but not in and of itself; it is important because it gives us something to focus on together. (DeMarco and Lister 2013, pt. 4)
DeMarco, Tom, and Timothy Lister. 2013. Peopleware: Productive Projects and Teams. 3rd ed. Addison-Wesley Professional.