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. 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!