As a developer, I absolutely love days like today. If the high-level "improves performance and stability and fixes bugs" changelog of Android 4.1.2 isn't good enough for you, how about we dive into the actual low-level source code commit logs Android engineers made into AOSP since 4.1.1_r1.1 (JRO03D) all the way through today's release 4.1.2_r1 (JZO54K). These commit logs are spread over probably 100+ repositories, so hunting for all of them manually would probably take you days.
Multi-user support is one of the few remaining things a desktop OS can do that Android can't. The "coffee table tablet" use case would greatly benefit from a multi-user setup, as would an enterprise user who wants to keep work and home separate. It's been a top 20 item on the Android bug tracker since the debut of Honeycomb, so there is certainly demand for it.
As we've seen from my previous experiments in sticking my nose where it doesn't belong, Google likes to leave breadcrumbs in shipping products for the astute observer to find, and the multi-user situation is no different.
Have you ever wondered what the AOSP source tree would look like if someone stitched together a video of every commit, update, and release? Ponder no more, friends, because YouTube user xcco3x has made that a reality. A visually amazing 21 minute reality, to be exact.
A little background info, per the description on YouTube:
The graph represents the source tree. Non-leaf nodes are directories and leaf nodes are files where their color represents the type of file.