One of the primary features of Mesa 7.3 is enhanced support for GLSL 1.20. Version 1.20 of the OpenGL Shading Language was released in 2006 and various bits of support have been added along the way, but Mesa 7.3 has more of the specification actually implemented. Support for the invariant keyword has been added along with the centroid qualifier and work related to GLSL arrays. There is no support for GLSL 1.30 / OpenGL 3.0 within Mesa 7.3.
The other prominent work that will make up Mesa 7.3 is the Intel DRI driver now supporting Direct Rendering Infrastructure 2 (DRI2) and the Graphics Execution Manager (GEM). The other mainline DRI drivers, however, don't yet support DRI2 or GEM memory management.
While those are the prominent changes in Mesa 7.3 Release Candidate 1, there have been more than 400 commits to the Mesa Git repository since the release of Mesa 7.2. Encompassed by some of this work are also a few ATI Radeon 3D fixes.
At the time of publishing the release announcement for Mesa 7.3 Release Candidate 1 has yet to emerge nor are the source packages yet available from Mesa3D.org, but they should be out there soon. We would suspect that the final release of the Mesa 7.3 development snapshot should be out within a few weeks followed by a stabilized Mesa 7.4 version in time for the Q2'09 Linux distribution updates. Mesa 7.3 will depend upon DRM 2.4.2 or newer.