Things I like in languages, and miss in others:

* First-class functions * Lambdas (And I should be able to do the same with them as with a regular function, unlike lambdas in Python, which are restricted to one line.) * Ability to define infix functions, like in Haskell or OCaml (although OCaml has some limits.) * Pattern matching * List comprehensions are nice, and could probably be implemented easily in the desugarer * Things like Scala's `val`/`var` of the `final` keyword in Java are nice. They allow you to introduce a certain amount of purity into a language. * Not quite a language feature, but if you're going to be generating executables, Tail Call Optimization is nice. * The Dynamic Proxy pattern is cool. Something like java.lang.reflect.Proxy would be cool.

Those are the things that I can think of right now that I often find myself missing in other languages that I can think of right now.

That assignment calculus paper looks really interesting. I'll have to take a deeper look at it when I have the time to.