The new stuff is mostly in the handling of Ext4 . And in particular in the handling of Blocks and Clusters through Bigalloc that will prevent swapping because Ext4 can handle Clusters up to 1 MByte as opposed to the former 4KByte.
That means less writing to the Bitmaps that will fit easier in the storage. + A lot of über smart stuff
The changes will not be backwards compatible.
In Btrfs the checking of Checksums has become faster and it now supports Qualcomms Hexagon-Architecture on the HW side.
Here is a (more) understandable overview of the changes.