The idea that Lisp's greatest strength - the inherent ability to transform to whatever you want it to be - is also its greatest weakness, since it prevents you from easily communicating with others what you intend - that is, one man's 'let (CL, Scheme) is another man's 'with (Arc).
Having ported my groovy Cells project to Arc, I can confirm that it was a pain where I sit that "only the names have been changed" in several cases. :) But pg gets the benefit of the doubt from me because I own both his books and he kinda rocks when it comes to Lisp.
The funny thing is that perhaps unbeknownst to you all the Lisp establishment is ragging on pg for Arc being too much like Lisp, you (and I a little) are hosed that he diverged from Lisp, so him being smart I would guess he is now ignoring everyone. :)
The only thing I can offer is that it totally sucks to have an installed base and not be able to change things cuz you will break all the code ofyour users. Only at times like this is an inventor free to change everything and anything, I guess pg did.