If you were hoping there was finally proper re-clocking / dynamic power management or other breakthroughs for this open-source NVIDIA Linux GPU driver, there isn't anything real exciting like that for end-users with Linux 3.15. The main changes to this drm-nouveau-next pull is the first stage of ongoing GPU fault recovery support, initial support for the Maxwell GPUs, and various fixes throughout the entire driver.
As explained in a Phoronix article a few days ago, the Nouveau support for Maxwell is very basic with 3.15. There will be kernel mode-setting support and there is also initial hardware acceleration support, but that work is dependent upon using the closed-source hardware firmware. GeForce GTX 750 series owners wishing to try Nouveau must first extract the binary firmware files themselves from the NVIDIA proprietary Linux graphics driver in order to have the acceleration support initialize. Besides having to jump through that step, there isn't any Maxwell support within the Nouveau Gallium3D stack. The ISA is very different with Maxwell and only a few days ago Nouveau developers got a triangle rendered with the driver. It will probably be a few more months before there is exciting Maxwell Nouveau support, but when using the NVIDIA proprietary Linux graphics driver, I'm really enjoying the Kepler successor.
Those curious about the bug-fixes that will land with the Linux 3.15 kernel, the drm-nouveau-next merge can be found via this Git commit.