On the F2FS side, there is improved performance by setting inline_dentry by default while also enhancing the discard issue flow and improving the memory management by another change. F2FS in Linux 4.9 also now allows preallocation of blocks for encrytped files and other changes and fixes. Enabling inline_dentry by default should help improve disk space usage and I/O performance for file-systems with many small directories.
A listing of the F2FS changes for Linux 4.9 can be found via this pull request.
With the EXT4 pull request it's "lots of bug fixes and cleanups."