As a 'seasoned' Rebol developer with some knowledge of the world outside, I'd be curious as to the utility/pitfalls of implementing Lisp-style macros in Rebol (and/or Red).
My understanding (always happy to revise) is that Lisp is able to preprocess code prior to evaluation/compilation, modifying that code according to the rules of the applicable macros. This can allow for more complex/expressive statements than otherwise, with efficiencies as those statements are expanded only once as a body of code is loaded. The macro rules themselves are relatively straightforward as the homoiconicity of Lisp allows the code to be manipulated by the same terms one would modify data.
As Rebol is also a homoiconic language, it stands that it too could accommodate macros between loading and evaluation with similarly expressive transformation grammar (though I'd posit that Rebol's more freeform evaluation model—statements not generally bound by parentheses—makes it harder to identify the portions of code to be transformed).
My own inclination is that in the main, macros aren't really necessary in Rebol—you can already create complex code structures that are fairly efficient, and where more efficiency is desired, Rebol has the capabilities to accommodate without significant loss of expressivity. On the other had, 'fairly' and 'no significant loss' could be considered weasel words and macros do offer an efficiency/expressivity bump, but at what implementation cost?
Also, as in Rebol you can load code prior to evaluation, it would be possible to implement a mechanism for load-expand-evaluate at the user-level permitting third-party macro implementations.
Red introduced a rudimentary macro system in version 0.6.2. I have used them effectively to create cross-compatible scripts that target both the Ren-C branch of Rebol 3 and Red.