The author of the ReiserFS filesystem for Linux (or GNU/Linux if you prefer) wrote:
Software Libre Takes More Than A License — It Takes A Design
Making the source code available to you is not enough by itself to bring you all of the possible benefits of software libre. Many file systems are so difficult to modify that only someone who has worked with the code for years finds it feasible to modify it, and even then small changes can take months of labor due to their ripple effects on the other code and the difficulties of dealing with disk format changes.
This is why we have a plugin based architecture in Reiser4, so that it is not just possible, but easy, to improve the software.
In writing this he managed to codify something that I have been thinking for a while. I was planning to add a plugin architecture for Chronicle Lite and this only fuels the fire. There are optional add-ons for the application that I wanted to release as self-contained plugins - would be wonderful to drop a JAR file into the application directory and have it find them auto-magically on startup.