Java binary compatibility
One of the things that my time at Symbian taught me was the importance of binary compatibility. In the end I was pretty good at thinking through whether any given change to a bit of C++ code would adversely affect users of the previous version of said code. Now I have to learn all the same stuff over again with Java, so that the release of the first version of my plugins doesn’t affect my ability to change them in future. Right now it looks like this might be a good starting point.