Java is a much more legible and flexible language. For example, in C++ you have to include header files and class files while in Java you only need to have a single class. A common misconception is that there would be a massive performance boost; Java uses a thin wrapper library around OpenGL, which is native to C++. It is possible, however, that there would be a performance boost down to the Java garbage collection which can slow down the program in general as well as the many libraries being loaded from Java. C++ is much more of a low level language too, which brings me back to my previous point that Java is more legible. Being a low level language means that it is closer to machine code, and harder to understand from a human's point of view. This would be a real pain for mod developers, plugin developers and even the Bukkit team. More code may also be needed for C++ so it would not necessarily be more "efficient". Java loads libraries from the Java installation itself which achieve many tasks quicker than the native libraries that are loaded by C++. Another huge problem with C++ is the portability; Java was made to be portable and perform the same or similarly on many different platforms, while C++ was made native to Windows. Lots of things in Java are fairly easy to do which much of the Bukkit platform in particular relies on such as reflection and run-time class loading. Another thing useful to developers is Javadoc, the standard created by Java which provides programmers with a clear documentation and saves them time overall. While C++ does have third-party tools for documentation generation, Javadoc is more encouraged through Java IDEs and has many more tools available for generation.
Many people's points are that just because more games are made with C++, it would be better. In actual fact, Java is beginning to overtake C++ in the most commonly used programming languages in the world and has just a marginal amount before it becomes top. Java is also constantly expanding and being revised so in my opinion it is the future. It is time for us to escape from the old ages of hardly legible code and jump into the new, refined programming languages of the future. Basically, my vote goes for no.
