WordPress 4.4 added some major things behind the scenes (REST API infrastructure) and really sweet things on the front end, like responsive imaging.
For all of you Jetpack users, Photon already supports the vast majority of new hotness with responsive imaging with the rest to come very, very soon.
Twenty Sixteen is now the present. You’ve seen the theme on this site for months now—it has had a pretty clean development cycle. There only thing that I particularly noticed as a minor coloring issue on the secondary color schemes which was quick to patch. I do wish they post format templates were a bit more interesting (or different at all), but that’s my only real critique. It looks good with great accessibility.
The new oEmbed support is pretty great. Need to drop in a post from any other site running WordPress 4.4+? Just drop the link into the visual editor, like https://wordpress.org/news/2015/12/clifford/ for the release post and it transforms into:
wp_die() since there are some nice little wins in there).