Seems like some nice improvements
- Continued work on F2FS by Samsung developers. The "Flash Friendly File-System" was just introduced in Linux 3.8, but initial testing has revealed it runs really great and also does well for SDHC storage. This may be one of the reasons (along with AArch64) why Google is quick to play with Linux 3.8 for Android.
- Power-saving improvements including the Zero-Power Optical Device Driver and more ACPI / ASPM work. There's also a new lightweight suspend mode.
- Improved ARM support, which I enjoy a lot because ARM performance is becoming very interesting.
- Major Linux audio/sound improvements.
- Google and other developers have been working to mainline all of Google's Goldfish, code for the Android emulator.
- DRM graphics driver changes, including early AMD Radeon HD 8000 series support, but aside from that and lots of internal changes to the various open-source graphics drivers, there isn't anything too outstanding as it concerns end-users.
- Hardware support improvements. There's the usual roundabout of new device drivers, expanded product support for existing drivers, etc. Among the 3.9 work includes new input drivers, Synopsys ARC700 CPU support, and IBM continues investing greatly in the Linux support for their next-generation POWER8 CPUs.