Very interesting. I've been playing around with Spark a little as of recently (Sinatra inspired minimalistic Java web framework). I think Mu may allow for some out-of-the-box functionality that Spark lacks.

At the end of the day, though, I'd be happy working in a Java framework wherein the experience didn't feel like I was forcing myself to write Java code. It's just such an uphill battle when the dev experience of (rails/node/django/webapp/web.py) is radically different from just about everything Java has to offer. The lack of a sane package manager, for one, is a major downer.