On a related note that has less to do with the essence of Scheme but a
lot to do with the essence of RnRS: there are some additional features
that one would expect from implementations of the recent RnRS reports.

The most significant change from R5RS to R7RS was the addition of a
library system. IIRC there have been implementations billed as R7RS with
little to no support for `define-library`. The main point of using R7RS
over R5RS is to be able to share portable code over the internet as
libraries, so lack of this feature is missing the main attraction of the
report (and with substantial ramifications for practical programming).

Likewise, R6RS without `(library ...)`, syntax-case macros, and the
standard condition hierarchy would probably not have much to do with the
spirit of R6RS.