It's a very neatly (nicely, had Nice not been the name of the language) done language with a strong research flavor. Type inference, multiple dispatch, anonymous methods (lambda, closure), and type parameters (generics) are the things that jumped out of the user's manual at me.

The syntax feels natural but is different enough from Java to make me stop and think. It uses the gnu.bytecode package that's part of Kawa and Qexo.

Guess what the following method does?

int->int f(int x)
{
  int g(int i) = i + x;
  return g;
}