To put it another way, the biggest problems with JSON aren't with the representation but with the semantics. S-expressions have no inherent semantics beyond those provided by the structure. Take this S-expression:

  ((:foo ("bar" 1))

and this JSON:

  { "foo": [ "bar", 1 ] }

and you'd think they have the same meaning, but they don't. The JSON decodes as "an object with one key, foo, which maps to an array containing the string 'bar' and the number 1." The S-expression decodes as 'the atom :foo is paired with the pair of the atom "bar" and the pair of the atom 1 and the pair of the nil atom." How to interpret the atoms :foo, "bar", and 1, as well as the meaning of their relative positions in the S-expression structure, is the actual hard part.

ETA: I just realized I pretty much repeated what an earlier comment said re: semantics. Sorry for the redundancy.