One fair comment that comes up frequently is that, with Java 8 out, Gosu's closure support is less of a win. Certainly true, so let me mention some other high-level features Gosu has that contrast favorably with Java:
* Type-safe reflection with feature literals
* Embedded classpaths in programs, including maven coordinates
* Integrated type-safe templates
* Pragmatic extension methods on the core java data structures via enhancements
* Null-safe operators
* Mixin support via the delegate keyword
* Implicit interfaces, like Go
* Type-safe templates built into the langauge
* Properties support (seems small, but it makes a big difference in day to day coding.)
And, of course, the Open Type System, which is essentially an API for introducing new types to the gosu compiler:
* Type-safe reflection with feature literals
* Embedded classpaths in programs, including maven coordinates
* Integrated type-safe templates
* Pragmatic extension methods on the core java data structures via enhancements
* Null-safe operators
* Mixin support via the delegate keyword
* Implicit interfaces, like Go
* Type-safe templates built into the langauge
* Properties support (seems small, but it makes a big difference in day to day coding.)
And, of course, the Open Type System, which is essentially an API for introducing new types to the gosu compiler:
http://devblog.guidewire.com/2010/11/18/gosus-secret-sauce-t...
All at least interesting, if not compelling.