One significant advantage is you can opt-in to sharing schemas - without requiring all consumers to have your schema. Like a lot of Amazon's internal data formats, Ion designed to support backwards compatible schemas as well (that is, adding additional fields does not break existing consumers).

It has isomorphic text and binary representations as part of the standard making debugging or optimized transport a config option.

The type system is significantly richer than JSON and maps well to several languages (internally Amazon uses it with C, C++, Perl, Java, Ruby, etc.).

S-Expressions.