The two main features introduced to the Btrfs file-system in Linux 3.10 is skinny extends and support for rebuilding of quota indexes.
Skinny extens is a new feature that's disabled by default but can be easily enabled via btrfstune -x. This new extent allocation tree format is able to make the allocation tree smaller by about 30%.
The quota index rebuilding is used if the quota indexes of the Btrfs file-system become out-of-sync or if the user decides to enable quotas on an existing file-system.
Aside from these two new Btrfs features, the other changes for this experimental next-generation Linux file-system for the Linux 3.10 kernel are mostly bug-fixes. The pull request in full can be found on the kernel mailing list.