Responding to Graham Glass, James Strachan says:

The fact that this [3rd party extension of the Java language] hasn't really happened yet and that generics, despite being available for many years, never gained popular adoption, makes me think that most folks are not that desperate for new bells and whistles inside the Java language and would rather see improvements in the core libraries, frameworks, tools, APIs etc. Most of Java's development has focussed on the platform rather than the syntax. Though now Java has a serious competitor in C#, maybe its time to redress the balance.

I think that this view is a bit optimistic, it's been noted before[1] that language features don't really catch on unless they're integrated into the language, e.g. Garbage Collection in C++. It's certainly possible to build a garbage collected system in C++, and there are commercial implementations (I think one prominent vendor is named Great Circle), but hardly anybody uses GC in C++. So the fact that hardly anybody's using third party extensions to Java isn't an indication that people don't want them; they either don't know they want them, or they don't want to use a third party extension. Especially given that Sun put so much effort into promoting "100% pure Java", it's not surprising that nobody's shown much interest in creating a "better Java". Sun's created a situation where their imprimatur is crucial for language features to be accepted by the wider audience of Java programmers. So development focuses on the platform, where extension is encouraged, rather than the syntax, where extension is discouraged.

[1] I couldn't find the original citation I was looking for, I think it was in an article by Paul Graham. However, while searching for it, I came across the article C++ vs. Lisp, where Brandon Corfman discusses his attempts to write C++ in a Lisp style. One quote that stuck out was

I will definitely say that after six years of C++ programming, I have tended to stay with what the language and library have given me.

which is exactly what I'm talking about here.