Adam Jackson's GLX-Rewrite is about simplifying and reducing the number of GLX implementations currently present in the X.Org Server. This GLX-Rewrite already has dramatically reduced the number of lines of code by tens of thousands of lines.
There's many GLX implementations right now within the X.Org Server that are redundant in supporting the common (DRI) open-source Linux graphics drivers. The old GLX back-ends included DRI1, DRI2, DRISW, Proxy, AGL, and WGL. With Adam's approach, there's just new AGL, EGL, GLX, and WGL back-ends present and understanding the driver interface is left up to Mesa rather than the X.Org Server. This work should also help in supporting the GLAMOR DDX driver properly.
It's hopeful the GLX Rewrite will be ready for the next release -- X.Org Server 1.15 -- but there's still issues to work through with the EGL back-end not being entirely agnostic and Win/Quartz/Proxy support not being done. X.Org Server 1.15 was already delayed to the end of this year this week with hopes that this GLX Rewrite and other interesting code will be able to be merged before the end of October.
For more details there are some rather basic PDF slides plus a video embedded below.