The design of DRI2, or the Direct Rendering Infrastructure 2, was a modest improvement over DRI (DRI2 design page) and noticeably to end-users it allows (accelerated) direct rendering to redirected windows. This becomes noticeable (or there the lack of) when using a compositing manager (Compiz) and watching a video or running an OpenGL application. DRI2 though has been dropped from the X Server 1.5 series due to its dependence on the TTM memory manager.
With Intel having jumped ship on TTM earlier this year and X developers complaining about TTM, the days of this hyped kernel memory manager from Tungsten Graphics look limited. Intel threw all of their weight into developing the Graphics Execution Manager as a new kernel memory manager. Some have been concerned about the GEM API being designed solely around Intel's needs, but it looks like it will be merged into the Linux kernel and will become the memory manager for new graphics driver work.
This abandoning of the TTM memory manager in favor of GEM has caused some problems for the Direct Rendering Infrastructure 2. DRI2 was designed around the TTM API and supporting this memory management API is a prerequisite for drivers supporting this infrastructure. As a result, DRI2 -- to some extent -- is forced back to the design table.
The DRI2 mastermind, Kristian Høgsberg, has announced on the X.Org mailing list that some of the DRI2 design decisions now need to be re-evaluated when switching from TTM to GEM. As a result, the current DRI2 code will be stripped away from the X Server 1.5 branch. We've been in a feature freeze for X Server 1.5 and this redesign work will take some time, so there's no hope in seeing a patched up DRI2 before X.Org 7.4 ships.
Input Hotness was already pushed back from X Server 1.5 as well as Multi-Pointer X (MPX). DRI2 is now the latest feature to enter the X Server 1.6 party. Also on the table for 1.6 is Distributed Multi-head X (DMX) and RandR 1.3. X Server 1.6 will ship with X.Org 7.5 sometime in 2009.