Apparently not only does this signal the start of major development in 2.5, but also means a lot of possibilities are promised after this is complete, for example.
Automatically generated user interfaces with lists of data properties.
Access to Blender data by plugins, for example in a render API.
Automatic python scripting access to blender data, without manually written code that might be duplicated elsewhere.
A uniform interface for native and custom defined properties.
Making all properties animatable and proxyable.