I'll be one of the few positive voices here, I guess. JSON Schema is pretty good. It's a relatively simple, extensible, pragmatic specification that supports validating the kinds of data that JSON can express. XML was killed by complexity, same as lots of technologies that preceded it, such as CORBA and SOAP. Developers don't like complexity. W3C tried to build an enormously complicated ecosystem of tools on top of X…
Edit: Of course, there is no direct equivalent of XSLT in the JSON world - which pretty much counts as a feature to me.