The J2SE sources really need some serious cleanup of the internals. The code is a total mess. Looked like it was coded by tens of different people with varying levels of coding experience and style (not to mention the poor design in many cases).
I'm not saying that you should refactor everything at once, but for any module that has seen more than ten bug fixes per release, consider refactoring the internals. Not only will you likely reduce the future number of bugs, but also focus on making the code more readable.
One of the most important things about open-source projects (in my view) is communication. Bad code breeds bad communication.
There is also the matter of the trustworthyness of the code. I feel .. uncomfortable .. building products on top of a framework that looks so messed up under the hood.
Just my 2 cents...