REBOL is a messaging language. Its main purpose is to provide
a better approach to distributed computing and communication.
I can not refrain from making the connection with Erlang while thinking that the concept is quite different even if the intention is the same in the words. To see later.
REBOL introduces the concept of dialecting
Dialecting is very interesting. I find this a paper, Language Oriented Programming of M. P. Ward about it. Have to read it.
REBOL implementations are intentionally kept very small
This is the aspect that I think keeps the whole very compact. I hope it's extensible, though.
The trends Rebol was trying to push back against, for its time...might seem passe now because some of it is (publicly) absorbed by those it influenced. It was a retaliation against things like XML that was flawed from the start or C++ name mangling. Or basically any other trend of that era where Carl perceived (rightly or wrongly, or half-right-wrongly) as people doing some "leap before you look" introductions of junk.
It had a reactionary bent to it. And that reactionary bent inspired Douglas Crockford (a Reboler back in the day) to make JSON.
Some of the more practical aspects that might have interested many people about it then, has likely been absorbed and improved upon other places. But it's good to know what it was before understanding where Ren-C is taking it.
I understand better now the introductory chapter of the French book on Rebol.
For the JSON format for example even if it represents a progress it still lacks the large number of Rebol type which makes it ambiguous with the usual languages, poor in types.