The key differences between Pygame and Pygame Reloaded can be found in the module layout and integration purpose. Where Pygame is a fully integrated package with nearly anything ready to use after doing a simple import and init, Pygame Reloaded uses a more modular approach, where each module needs a seperate import and a few more lines of code to get anything ready to use.
Especially the SDL-related modules try to wrap all portions of the original C API in a 1:1 manner, using Python's full power.
What looks like more work gives you, the developer, a fine-grained control about what to use and what to ship with your application or game. As the package and API layout, Pygame Reloaded's build system is very modular, allowing you to build and distribute only the necessary coupons for your application or game.
Making use of basic abstraction and interfacing rules, Pygame Reloaded offers a variety of base classes, where third party modules can hook up and interact with certain other parts of Pygame Reloaded. This gives you, third party libraries and anyone else more flexibility for interoperational tasks and information sharing.