The json spec does not allow commas. Although there are jsom supersets that do.

> Comments allowed or not

The json spec does not allow comments. Although there are jsom supersets that do.

> Multiple kinds of date serialization conventions

Json spec doesn't say anything about dates. That is dependent on your application schema.

> Divergent conventions about distinguishing floating point types from integers

This is largely due to divergent ways different programming languages handle numbers. I won't say jsom handles this the best, but any file format used across multiple languages will run into problems with differences in how numbers are represented. At least there is a well defined difference between a number and a string, unlike csv.

> Duplicated key names tolerated or not

According to the spec, they are tolerated, although the semantics of such keys is implementation defined.

> Different string escaping policies, such as, but not limited to "\n" vs "\x0a"

Both of those are interpreted as the same thing, at least according to the spec. That is an implementation detail of the serializer, not a different language.