- They both use POT (plain old text) as their encoding
- The basic datatypes are strings and numbers. JSON adds booleans and nulls - Yay for progress!
- The only schema metadata supported is field names
CSV has stood the test of time extraordinarily well. According to good 'ol Wikipedia it's been around since at least 1967 - that's over 40 years!
And CSV is still well supported whereever tabular data is used. Let's see if JSON is still around in 2035... I suspect not, because the half-life of technologies is a lot shorter these days. (Maybe CSV is the stromatolite of file formats!)
It would be nice if JSON had a standard schema notation. (There is JSON-Schema. It copies the XML Schema idea of encoding the schema in JSON. It remains to be seen whether this makes it as easy to use and popular as XML Schema has been. 8^)
YAML seems to have a lot advantages over JSON as a rich textual format. For instance, it has minimal use of quotes, and a richer, extensible set of datatypes including timestamps and binary (WKB, anyone?). But it's going to be pretty hard to dislodge JSON, which is solidly entrenched for all the wrong reasons.