Have you ever used Array#map in Javascript (or any dynamic language)? What about filter or reduce? Those are useful things not possible in Go.

Well, technically they are (whipped up a quick example): http://play.golang.org/p/e2qkKN4VY1

But it requires the use of interface{}, which reduces the usefulness of the type system, and you still can't do (what I guess must be crazy) stuff like return a slice.