Overlay2 support is really nice. I had to use the ZFS driver for it and boy does it pollute your zfs list output.
Also pretty excited about the user properties, it's gonna make initramfs mounting a lot easier over the somewhat buggy parsing of the output of zfs list. Same for encrypted home directories, now I can set a custom user property on them to easily enumerate what my PAM script is supposed to mount/unmount with the user password instead of hardcoded paths.