Edit-in-place, usability, and easy to create custom content types made Plone 2 popular and long lived
Plone introduced the concept of edit-in-place, and Plone 2 was the first feature-complete and stable version of this radically different content management system. There was no separate administrative back end - instead editors navigated through a filesystem-like hierarchy to the page of interest and just clicked edit. This was a great user experience for editors, and Plone began to take off.
Plone's founders had another brilliant idea, which was to bake accessibility into the system. They made out-of-the-box Plone 2 compliant with accessibility standards, an effort that has continued on all successive versions. Thus began Plone's commitment to accessibility for both users and editors.
A Content Type Framework
These powerful ideas made Plone great for users, but it was messy for developers to add new types of content in addition to the pages, events and news items that came out of the box. The community stepped up and created Archetypes, a framework for content types.
Content types were so well structured that it was possible to generate them automatically from an Entity Relationship Diagram. Plone Bootcamps were held and hundreds of people learned to do Plone development.