Ask HN: Why is PLT Scheme's macro system so great?

According to Wikipedia:

The most remarkable feature of these remains the macro system. It provides far more expressive power than Lisp's S-expression manipulation system, Scheme 84's hygienic extend-syntax macros, or R5RS's syntax rules. Indeed, it is fair to say that the macro system is a carefully tuned API for the compiler. Using this compiler API, programmers can add features and entire domain-specific languages in a manner that makes them completely indistinguishable from built-in language constructs. For example, both the class system and the component system are nothing but macro libraries.

What I don't understand is:

* how is this more powerful than Lisp's S-exp macro system and

* haven't people created object systems on top of Lisp's s-exp system?

Thanks!