Could you elaborate on Scala's TCO support? I was under the impression that only self-tail calls are eliminated and only if the stack trace would be provably unused. Doesn't the JVM require stack traces? Can you use CPS in Scala?

I see now that Scala can encode GADTs in case classes. What mechanism is available to encode polymorphic variants? Particularly, I am interested in writing matches over unions of polymorphic variant types.

I disagree that 2 and 3 are the same (structural module subtyping, nesting, inclusion differs from applicative functors). I believe you can achieve mixins with OCaml through the object system but it's not clear to me that this addresses functor signature checking.

Another Scala bonus: 5. Objects can implicitly be null.