- Avoid feature overload: Larry Wall’s oft-cited saying “there is more than one way of doing things” to me is the very definition of bad language design. Scala and Clojure might suffer from this problem (disclaimer: I still don’t know these languages well enough for an informed opinion).
- Typing method parameters is great: It allows one to do static checking and documents what’s going on. Languages with algebraic types (ML, Haskell) tend to have even more custom types; they even wrap the type where in Java, one would probably use naked strings. A great practice.
- I would also argue that (a simple version of) closures should completely replace inner classes. They are immensely confusing to newcomers and not needed if you have closures. Smalltalk and (the partially Smalltalk-inspired) Ruby show that closures and object-orientation are not antithetical.
- Groovy++ looks interesting, sometimes it seems to stray too far from Java (why in hell are semicolons optional?). [As an aside: Groovy++, please get a proper home page, for those of us who are banned from Google Groups.]
- ABCL (Armed Bear Common Lisp) is also worth checking out. I love their tongue-in-cheek slogan “The right of the people to keep and arm bears shall not be infringed!”. Like Clojure, ABCL is Lisp, but closely emulates Common Lisp which means it is a simpler language.
Original Sin” sketches what a Java without primitive types could look like.