JSON Feed is a lot like RSS, except instead of XML it's formatted with JSON. It's a rather new spec, introduced just last week, but it's been getting some major coverage and a number of applications have already started supporting it.
Since Postleaf was built for the modern publisher, I decided to add support for it in alpha 5. You can check out the relevant commit to see how easy it was to implement.
It's just like John Gruber says, "easier to generate, easier to parse."
I expect to see a lot about JSON Feed in the coming months. I'm even happier to report that, within days of the spec being released, Postleaf is well-prepared!