What makes Agile / Lean Work?

I have a theory.

Agile and Lean development practices are not magic, they are also not revolutionary. And it doesn't really matter which flavor you choose. What makes Agile and Lean work is that they simply provide a recipe for following what the management gurus have been saying for the past century. If you want to increases productivity give regular, actionable, feedback.

As an example, take a podcast from Manager Tools on Over Assigning and Delegating Work which breaks down Peter Drucker's advice on how much work to assign to employees.

The key to doing this well as an organization is for all the employees, at all levels, to develop excellent skills at prioritizing work. Even though there is always more work than can be done, the goal is to in each day, week, month ... Do what is the Most Valuable for the company at that time.

To me, this explains the power of the retrospective. Having these regularly (and not like your "performance reviews" are regular - one a year under duress) is how you gather actionable feedback on how well you prioritized your work and goals. A very basic question to ask in each retrospective - is what you did valuable? Was it the most valuable?