While there are many new features to the Linux 4.6 kernel, the Btrfs changes for this next kernel cycle are on the lighter side.
Btrfs maintainer Chris Mason noted in Monday's pull request, "We have a good sized cleanup of our internal read ahead code, and the first series of commits from Chandan to enable PAGE_SIZE [greater than] sectorsize. Otherwise, it's a normal series of cleanups and fixes, with many thanks to Dave Sterba for doing most of the patch wrangling this time."
The Btrfs changes for Linux 4.6 just change around roughly one thousand lines of code. Hit up the pull request for more details on the fixes and clean-ups.