The main feature of the latest intel-testing code push is the HDMI Stereo 3D support. There's been experimental HDMI Stereo 3D Linux patches for Intel hardware going back to 2012 for supporting the HDMI specification surrounding a standardized 3D display format. Changes are needed to not only the kernel DRM but also libdrm and optionally to the intel-gpu-tools for testing purposes.
Other changes include manual boost/deboost for Intel hardware with a GPU Turbo mode, fixed clock read-out support for Valley View / Bay Trail, tons of little fixes and improvements for Valley View / Bay Trail, powerwell support for the legacy VGA plane, DisplayPort improvements, Haswell mode-set sequence improvements, Haswell DDI improvements, and tons of other small improvements and fixes.
Those wishing to help out in testing this latest Intel DRM graphics driver code can find details via the intel-gfx mailing list. Look for this code to land not in Linux 3.12 but for Linux 3.13!