Today, with a great sense of relief, I committed the second of my major JSON patches for this release cycle, just beating the imminent close of the current commit-fest.
Most of the attention has focussed on the new JSON processing functions, which you can see details of in the dev docs. But the most important part in my mind is really the less visible part, the scaffolding on which these new functions were build. Essentially this took the original (and very good, for its purpose) JSON parser that Robert Haas wrote for release 9.1, which was stack based, and turned it into a classic recursive descent parser. In doing that it provided the ability of functions calling the parser to supply callback hooks for certain parsing events, such as the beginning or end of a JSON object or array. It's kind of the JSON equivalent of an XML SAX parser. That means it's quite easy to construct new JSON processing functions without having to worry at all about the nitty gritty of JSON parsing. I expect to see a number of JSON processing extensions based in this (and I expect to write a few myself).
I'm hoping to give a talk at a conference later in the year about how to build these processing functions.