The soon-to-be-released Mesa 9.2 comes up short for some Linux enthusiasts and open-source desktop users with regards to:
OpenGL - Mesa 9.2 only has OpenGL 3.1~3.2 support for the key Radeon, Intel, and Nouveau graphics drivers. The latest Khronos specification is OpenGL 4.4 as of this month, which won't be coming to Mesa anytime soon. The latest hope is to have open-source OpenGL 3.3 support by the end of the year while GL4 support isn't likely until the 2014 Mesa releases. The proprietary AMD and NVIDIA drivers on Linux already support OpenGL 4.3/4.4 and the Windows proprietary driver from Intel is also GL4 compliant.
RadeonSI & LLVMpipe OpenGL - While the R600 Gallium3D driver supports OpenGL 3, the "RadeonSI" Gallium3D driver for the Radeon HD 7000/8000 series graphics cards only fully enables OpenGL 2.1 support. GL3 support for this newer Radeon Gallium3D driver is still actively being pursued. The LLVMpipe software driver fallback also only advertises OpenGL 2.1 but almost has OpenGL 3.0/3.1 support. Hopefully for the next Mesa release these increasingly-used drivers will see the light of GL3.
OpenCL - The OpenCL language support via the "Clover" Gallium3D state tracker is still less than ideal. The Radeon and Nouveau drivers have been working towards Gallium3D OpenCL and some GPUs can withstand bitcoin mining on the open-source stack, but for most users interested in real-world CL workloads, the Gallium3D support isn't yet enough.
Performance - The Nouveau driver is not faster than the NVIDIA Linux (or Windows) driver, the Radeon Gallium3D drivers are not faster than AMD Catalyst, and the Intel Linux driver isn't faster than the Intel Windows driver. The Mesa/Gallium3D drivers still have performance that comes up short of the proprietary competition, but at least the situation has been improving.
Sadly, these missing/incomplete features haven't changed much since Mesa 9.1. What missing features or open bugs are currently making your Mesa driver experience less than ideal? Share with us in the forums.