Just a thought...
Since there's made a distiction between extensions and plugins I notice some confusion.
Wouldn't it be better for instance to make an extra tab in the backend called extensions?

There is no distinctions. There are just two folders. zp-core/zp-extensions for the official plugins and plugins for third party ones.

Besides that plugins are extensions...;-)

Zenphoto "extensions" are shown in [b]bold face[/b]. Third party "plugins" are in normal face. The only distinction is where the plugin came from and whether we Zenphoto developers can support it. All are labeled "plugins" when used within Zenphoto.