We've already been covering the Linux 3.12 kernel changes for the past two weeks while the merge window has been open (and prior to that with covering the interesting -next Git trees and other early mailing list activity discussing to-be-merged patches).
If you didn't already read An Overview Of Linux 3.12 Kernel Features, I would encourage you to do so as it concisely lists all of the interesting bits that made it for Linux 3.12 -- at least from our perspective with the enthusiast and desktop hardware angle. There's lots of open-source graphics driver improvements, the continued evolution of Linux file-systems, more ARM work, and various new/updated hardware drivers.
You can also read Torvalds' 3.12-rc1 announcement but it's not terribly interesting for end-users or even enthusiasts. Of the merge window activity, 73% of the code churn was within drivers, 12% within the architecture area, and 6% for file-systems while the remainder was scattered among the other subsystems. The feature features mentioned by Linus were scalability improvements, cleaned up TTY locking, and dentry refcount scalability.
We'll have the usual Linux 3.12 kernel benchmarks in the coming weeks. Already on Phoronix I have shared Intel (Haswell) Iris Pro benchmarks and early file-system tests.