>>8Very little is portable across lithps. Even the very idea of macros is nonportable: scheme officially didn't have them before r4rs, and they didn't really come in until maclisp. And even the more lithpy scheme macros systems like sc and er/ir aren't exactly the same as defmacros.
Just about the only thing that is portable is the idea that lists exist and are the core datastructure, but even the names of the primitives vary from lithp to lithp, although it's almost always cons, car, and cdr nowadays.