In regards to Intel's open-source DRM i915 driver for Linux 3.15 I've already written about Intel tackling DRRS support, Broadwell GPU improvements, and lining up more graphics driver changes. Daniel Vetter of Intel's Open-Source Technology Center has now written a personal blog post covering some of the changes that will be found in the Intel DRM driver merge for Linux 3.15.
The highlights covered by Daniel for "neat" i915 DRM 3.15 changes include per-process address space support (currently limited to Ivy Bridge and Haswell but Bay Trail and Broadwell support is coming), fine-grained display power domain handling, runtime power management infrastructure work, support for inheriting the firmware frame-buffer as another step in Fastboot support, a lot of Broadwell patches, improved support for frame-buffer compression, 5.4GHz DIsplayPort support, generic DisplayPort aux helpers, and large cursor support to benefit HiDPI displays. For Intel's 4K display support, they now support 5.4GHz DisplayPort but they don't yet support multi-stream support (MST) as most 4K DisplayPort screens expose themselves as two displays to the driver.
More details on these Linux 3.15 Intel graphics changes can be found via Daniel's blog.