I agree with you -- I wasn't refuting your point, but rather noting that it's worth considering as a major usability issue for the language, rather than strictly a technical decision. (I meant to say, "One can argue ...", but hadn't had my tea yet. I can see why you read it that way.)

I had Hexstream's glib "The solution is to use prefix notation. Problem solved!" reply in mind, actually: in that case, the language trades a syntactic ambiguity for pissing off everyone who expects to do basic arithmetic without major problems, which will disproportionately be people new to the language.

Cavalierly dismissing this is hardly a solution. If I were new to Lisp, and trying to use it for something based on relatively simple arithmetic (say, totaling a categorized spending log), and a question about why "total + val" didn't work was answered with a digression about parsing, I would be pretty annoyed. If the person talking up Lisp were to then go into a spiel about how Lisp is great because you can extend the language to express things in their own terms when I couldn't even use arithmetic, I would probably assume they were full of it and write off Lisp entirely.