I've been playing around inside the closure-compiler in my free time recently and the thing that struck me most after years of working in other languages (Ruby, JavaScript, C#) is that Java is a relatively simple language. I have yet to find myself spending any significant amount of time wondering at what a given snippet of code does.
[edit] I should note that I don't have much to compare the closure-compiler too, so it might be that this is the result of a very small sample set.