There's a hidden gem in Nexus 9, which was announced by a short sentence in the middle of a reply in a long mailing-list thread:
"No proprietary binaries are needed for Volantis. The proprietary vendor binaries are on a separate 'vendor' partition."
Until now, in Android devices, the proprietary device-specific files that live underneath Android itself were stored in the same /system partition as the Android files.
This made sense from the point of view of software architecture, but it had a major drawback in the Open Source world: in order to distribute a functional system image of Android, it was necessary to also distribute those proprietary device-specific files, since those files were aggregated into the same distribution medium.
Starting with Nexus S, those files had been somewhat available, with two caveats: not all files were available for all devices, and the files that were available were controlled by licenses that allowed the most common use cases but didn't give the same freedom that can be expected for Open Source components.
On Nexus 9, things are different: those proprietary device-specific files are stored in a separate partition. As a result, it is now practical to distribute functional versions of the Android system without having to distribute or copy those proprietary files. Therefore it becomes possible to enjoy the freedoms associated with Open Source in a broader range of situations, including (e.g.) commercial distribution.
While Android has always been distributed under Open Source licenses (i.e. in the world of lawyers), this brings it closer to the spirit of the Free Software definition in the real world (i.e. in the world of hackers).
This makes me happy, as this is the conclusion of a task that had been started 3 1/2 years ago with Galaxy Nexus, and in which I had been closely involved when I worked on AOSP. Chances are, this is probably also the last aspect of Android to get released in which I've been closely involved while at Google.
As usual, I'm not speaking on behalf of any past, present or future employer. Also, I'm not a lawyer, and this is therefore obviously not legal advice.