Ordering of releases in json
One thing that the json API lacks compared to the xmlrpc API is the ordering of releases. In the json API, the releases are keys in a dictionary, and hence unordered.
The tarballs have upload times, but a release can have multiple tarballs, and also if I'm not mistaken you can upload a tarball to an old release.
The xmlrpc API has package_releases(package_name, show_hidden=False). The json API could have a package_releases key with a list of releases. I don't know how to handle hidden releases.