Improvements to the Advanced Host Controller Interface (AHCI) code within the Linux kernel allow for more code to be shared amongst drivers for this SATA disk interface. In particular, a lot of embedded platforms were implementing AHCI controller support and were doing so in a variety of different ways. With Linux 3.15, libahci and the AHCI platform code have been improved so that these platform drivers can share more common code. There's also been the introduction of some new drivers to replace older drivers.
Another noteworthy libata change is making the power management operations asynchronous.
The libata updates for Linux 3.15 were already merged and more details can be found via this Git pull.