I looked at it more. It has both a string type (which has \xHH escapes oddly specified as code points), and a binary type (which is specified as base64 in text

I looked at it more. It has both a string type (which has \xHH escapes oddly specified as code points), and a binary type (which is specified as base64 in text format -- this is bad because base64 isn't human-readable despite being ASCII). The 2 different types makes it unsuitable for the use cases I'm targeting QSN at, which deal only with byte strings, which may be encoded in UTF-8 (like kernel APIs). I don't see what Ion adds over MessagePack, CBOR, or several other such formats... but that's a different discussion :)