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. However, thanks to Al Sutton, you can check them out all in one place.
Be prepared for lots of code jargon and incomplete git commit messages, which probably won't mean much to most of you.