It’s a superset of JSON with an isomorphic binary encoding, additional data types (blobs, s-exps, timestamps, symbols, etc.), better number handling, annotations, and the ability to pre-share symbol metadata for more efficient binary encoding (similar to how protobufs encodes fields, but optional).

You can write Ion by hand (like JSON) and share it without a schema (unlike protobufs). There’s fewer ways to express values than YAML, but more data types.

Having S-exps is convenient for writing DSLs in a data language that’s easily readable from other languages.