code/data duality = homoiconicity
There are many languages which do not use sexps but are homoiconic.
See "examples" at https://en.wikipedia.org/wiki/Homoiconicity
Yes, most of the examples look very limited compared to Lisp but there is Rebol / Red (http://www.red-lang.org/) which is atleast as powerful as Lisp in terms of homoiconicity (as well as in other things), if not more. Red/Rebol don't use sexps, don't look like this (((())))), are highly dynamic and homoiconic.
See "Homoiconicity in Rebol" in https://en.wikipedia.org/wiki/Homoiconicity (well, no good examples over there which shows Rebol's features properly.)
> RE the learning curve: Do you think the APL family can exist without the terse syntax?
So yes, I think that the APL family could exist without the terse syntax, but well hey, everything has its tradeoffs and all this is subjective - everyone has his own thinking - there might be someone in this world who in love with the APL syntax :D