Lisp introduced us to: code == data
Clojure introduces us to: code == concurrent high-performance immutable data
(I hope that parallel being the only way to gain additional performance should be obvious by now.)
I'm not knocking PG here - I just don't see Arc having any real differences from all the other lisp variants. There's nothing revolutionary about it. Which is fine - it is a 100-year language, not a revolutionary language.
I just wonder what the odds are that anyone remembers it in a decade. People use langauges that are unique, and as far as I can see, Arc is not all that unique (unless you're the sort of person who becomes highly excited about the minutia of Lisp semantics)