Similar to Nine Good Things About Mesa 9.0 and Nine Reasons Mesa 9.0 Is Disappointing For End-Users, this list is looking at the top positive traits to Mesa 9.1.
OpenGL ES 3.0 - Mesa 9.1 introduces initial support for OpenGL ES 3.0. This is the first mobile/embedded OpenGL specification update in years since OpenGL ES 2.0 and incorporates some features from the desktop OpenGL 3.x/4.x specifications. OpenGL ES 3.0 support in Mesa so soon can be directly thanked to Intel and for Ivy Bridge they are one of the first official OpenGL ES 3.0 products. Though to date there isn't much software that's directly dependent upon OpenGL ES 3.0 functionality.
OpenGL 3.1 Core For R600g - While not new for the Intel driver, the R600 Gallium3D driver with Mesa 9.1 officially supports an OpenGL 3.1 Core Profile context. This GL 3.1 core profile support is for the Radeon HD 2000 through Radeon HD 6000 series. As part of this is GLSL 1.40 for R600g.
New OpenGL Support - While OpenGL 3.2/3.3 isn't fully and officially done for Mesa 9.1 (hence why there wasn't a major version bump to Mesa 10.0), support is getting close. There's support for a number of new desktop OpenGL extensions from the OpenGL 3.x and 4.x standards as part of this six-month update. There's several new extensions supported such as GL_EXT_color_buffer_float and GL_ANGLE_texture_compression_dxt5.
Radeon X1000 Series MSAA - For those using an old Radeon X1000 (R500) graphics card, Mesa 9.1 is the first release to finally support multi-sample anti-aliasing. MSAA has been worked on for both R300g and R600g while for R500 hardware it was finally deemed stable and good enough to enable the support by default. The multi-sample anti-aliasing level can be easily toggled via the GALLIUM_MSAA environment variable.
Performance Improvements - There are some performance improvements to see out of some of the Mesa/Gallium3D drivers in Mesa 9.1. For more information see Intel Sandy Bridge Looks Good On Mesa 9.1, Mesa 9.1 Results Are Mixed For Radeon Gallium3D, Mesa 9.1 Delivers Faster Intel OpenGL Graphics, and Five-Way NVIDIA GeForce Comparison On Nouveau.
Intel Haswell Support - Support for Intel's Haswell processors that will be released in the months ahead should be in good shape with Mesa 9.1. You'll also need the latest xf86-video-intel DDX and Linux kernel (Linux 3.8+) for the best open-source graphics support on Haswell.
Intel Floating-Point Textures By Default - The Intel Mesa DRI driver now enables support for floating-point textures unconditionally. Up to now the support was hidden behind a --enable-texture-float configure switch at build-time, but Intel found it safe to enable by default. OpenGL floating-point textures has been a legal concern for Intel and the other GPU vendors over patent/IP concerns. In talking with Intel Linux developers at FOSDEM earlier this month, they basically received legal assurance that they can safely enable floating-point textures without falling into legal hell. The Radeon and Nouveau drivers though still require the compile-time switch for enabling this GL3 functionality.
Improved Radeon HyperZ - When paired with a modern Linux kernel Radeon DRM, the HyperZ support within the R600 Gallium3D driver is finally in good standing. After sorting through problems for a long-time, open-source Radeon developers finally got the HyperZ support fixed-up. HyperZ has the potential to boost the graphics card's performance.
RadeonSI Gallium3D - According to Alex Deucher and others, the "RadeonSI" Gallium3D driver for supporting the AMD Radeon HD 7000 series is finally in okay shape for handling compositing and most OpenGL games. Last time I tried the latest RadeonSI Gallium3D driver with the Linux kernel, it still was a mess. However, I'll be testing Mesa 9.1 in conjunction with the Linux 3.8 kernel to see if things are finally working out decent for this latest-generation AMD graphics hardware.