Transform feedback is needed for OpenGL 3.0 compliance, but up to this point in Mesa it's only been partially implemented (see the latest Mesa GL3 status update). While parts of OpenGL transform feedback have been supported in Mesa, there's some interface changes needed so that this GL3 feature will work properly with the Gallium3D infrastructure and drivers. From Marek's email, "The old interface was unusable for OpenGL and had to go away."
GL_EXT_transform_feedback is the main transform feedback extension, but this work is also necessary for supporting GL_ARB_transform_feedback2 and GL_ARB_transform_feedback_instanced. These two later extensions are necessary for OpenGL 4.0. The OpenGL Wiki describes the feature as "the process of altering the rendering pipeline so that primitives processed by a Vertex Shader and optionally a Geometry Shader will be written to buffer objects. This allows one to preserve the post-transform rendering state of an object and resubmit this data multiple times."
The patch published by Marek last night amounts to about 100 lines of new code.
For more information on OpenGL transform feedback, see the GL_EXT_transform_feedback specification.