Plone 3 benefited from the arrival of a new way to develop Python web applications
In Plone 3 the pendulum swung toward adding spiffy new developer features based on the exciting new Zope Component Architecture, a Python framework for supporting component-based design and programming. The ZCA brought concepts like interfaces, views, subscribers and adapters, and Plone developers went a little crazy with these new possibilities (rumor has it there is a 7-way adapter in z3c.form)... But by adopting these technologies, Plone developers were able to write relatively normal Python code which brought improved maintainability and reproducibility.
A New Direction
In response to all these changes, the Plone community held a Strategic Summit in 2008 at Google headquarters. This event generated oodles of ideas for Plone improvements. One important outcome was a commitment to developing a through-the-web content type creation tool. This eventually led to Dexterity, a new and improved content type framework which began development during the Plone 3 era.