Open Source Community. The number of excellent open-source tools for Java is staggering. Look at HSqlDb, BeanShell, Eclipse, Recoder, JGraph, Tomcat, JBoss, and many more. More importantly, the Java community has proven much more interested in doing it the open-source way.
Eclipse. Already mentioned, but it deserves a point of its own. Eclipse is a better IDE than VS.NET!
Less Native Code & more code reliability. .NET still has some weird crashes. Despite much improvement, I have still experienced DLL-Hell light.