1. Macros. The Common Lisp version has this problems, but the Racket guys have hygenic macros, which are much better. I have probably not fully understood them, but the most important thing is imho to use lexical binding even inside macros.

2. Continuations. Higher level than closures? The real understand of continuations comes from the implementation imho. You implement your stack frames as a garbage collected tree data structure, instead of the C-way of a memory blob.