OpenGL 3.1: While there isn't yet full OpenGL 3.1 compliance throughout Mesa and all of the capable hardware drivers, at least it's pretty much there for Intel's DRI driver. Intel does most of the OpenGL enablement these days within core Mesa and their driver, so it's no surprise that they are first. Unfortunately though not even the LLVMpipe software-accelerated driver is at OpenGL 3.1 compliance (or even 3.0 for that matter). Originally, OpenGL 3.1 support wasn't expected until early 2013 so this release was originally known as Mesa 8.1 until a few months ago. The other drivers (namely Radeon and Nouveau) meanwhile with Mesa 9.0 have reached OpenGL 3.0 compliance for more hardware and picked up support for various other GL extensions, some of which are from OpenGL 3.1 and newer.
New Hardware Enablement: With Mesa 9.0 there's now the RadeonSI Gallium3D driver for supporting the AMD Radeon HD 7000 series graphics processors. Unfortunately this support still comes up short for the nine month old hardware and isn't yet in a proper state for end-users in terms of actually having working and reliable OpenGL support... At least with Mesa 9.0 there's also AMD Trinity APU support, which does work since it's not a real Southern Islands GPU so the support is less work and builds upon the R600g driver. On the Nouveau driver side, there is basic GeForce 600 Kepler series support. With the Gallium3D "NVE0" Kepler support you can at least run basic OpenGL games, but the Nouveau Kepler support still leaves a lot to be desired in terms of re-clocking for better performance and other features not yet being implemented. For old NVIDIA hardware, Mesa 9.0 has a new NV30 Gallium3D driver. On the Intel driver side, they continue working on enablement for next year's Haswell processors.
Multi-Sample Anti-Aliasing: The Radeon Gallium3D driver finally supports Multi-Sample Anti-Aliasing. MSAA has been incredibly common to the proprietary graphics drivers and to all Microsoft Windows platforms for many years now while the open-source Mesa drivers are finally catching up... R600 Gallium3D can now handle MSAA across the different hardware generations. The Intel driver also has improved MSAA support.
OpenCL: After being tackled on and off for several years, the Clover state tracker for Gallium3D was finally merged to master this cycle. Clover provides OpenCL over Gallium3D. The Radeon and Nouveau developers have both been working towards OpenCL on their open-source drivers with some basic success. The OpenCL Gallium3D support still doesn't come close to the proprietary drivers and their fully-functional state, but at least they're progressing in the right direction.
VDPAU State Tracker: The Gallium3D VDPAU state tracker for providing GPU-based video playback acceleration via shaders reached a "completed" state for Mesa 9.0. While it's considered complete, the Video Decode and Presentation API for Unix support within Gallium3D is presently limited to MPEG1 and MPEG2 formats.
Performance Improvements: There are some performance improvements to Mesa 9.0 relative to Mesa 8.0 as various Phoronix benchmarks have shown while Mesa 8.1/9.0 was in development. More benchmarks will come though in the next few days from the Intel/Radeon/Nouveau drivers to see where things are at now with the official Mesa 9.0 release and next is Mesa 9.1-devel.
Left 4 Dead 2 / Source Engine: The Intel DRI driver can now handle running the native Linux client of Valve's Left 4 Dead 2 game. Plus other Source Engine games should be able to run too on the Intel driver when using Mesa 9.0 along with installing the third-party S3TC library and ensuring Mesa is built with the patented floating-point textures support. The performance is alright for L4D2 with the Intel Mesa driver.
Intel i915g Gallium3D: Google worked out sRGB textures for the Intel Gallium3D driver. This change allows the i915g driver to advertise OpenGL 2.1 compliance, but it's basically the last of new features for this unofficial Gallium3D driver that supports the old Intel i915/i945 chipsets.
Various Other Features: Other smaller work landed for Mesa 9.0 like Intel hardware context support, many bug-fixes, and other small enhancements to core Mesa and the different hardware drivers.