I think that syntactical/structural forms are the way of capturing in syntax recurrent programmatic patterns so they stand out. So a language that captures patterns syntactically absent from other languages is certainly idiomatic. Perl is very aggressive in capturing patterns with its syntax so it is certainly idiomatic. A language can also be very idiomatic by its very repetitive syntax like Lisp and its "clipped nails". I would use the expression syntactically idiotic here to qualify Lisp.