I'd argue that all of those are either problems in XML, or considered features. Datatypes and comments would give room for extensibility, which is against what

I'd argue that all of those are either problems in XML, or considered features. Datatypes and comments would give room for extensibility, which is against what JSON is intended to be. JSON should be instantly readable by any system, it shouldn't need a schema describing what all the datatypes and comment annotations mean first (because that's inevitably where comments would lead). Appending is a problem in any structural format, XML is not immune. Binary data is a problem, I will agree. The real benefit to JSON is that it's so lean and easy to parse. I can get a hashmap or a plain object out of some JSON text in a few lines of code in almost any language, the same can't really be said of XML, especially not the kind of XML usually used as a data interchange. Additionally, that lean-ness means it has significantly less data to transfer, I've seen JSON versions of data be 1/10 the size of the XML equivalent, which can matter at the far end of the spectrum. The real strength of JSON, though, is arrays. List structures have always been a major weak point of XML, as there's no way to do it without a million tags. God help you if your list has anything other than a primitive in it.