I was wrong about S-expressions. S-expressions are roughly the right complexity, or even a bit too complex, depending on how you look at it.
JSON is way too complex. JSONS assumes that you have an object/record structure (labels and values), and gives you both objects and arrays with which to build tree structures.
JSON is one of the most dominant tree-data formats on the web because of the awesome balance it has between complexity and simplicity. JSON fits the bill nicely.
Actually... it's one of the most dominant tree-data formats on the web because you don't have to bundle a parser with your client and then figure out the idiosyncrasies between your serialisation model and your language's object model, the browser gives you one for free and JSON is your language's object model. It's a lot more fiddly to deal with in languages which don't have a K/V map as the primary object model.
JSON is way too complex. JSONS assumes that you have an object/record structure (labels and values), and gives you both objects and arrays with which to build tree structures.