Key changes for Btrfs in Linux 3.7 include hole punching, send/receive fixes, improved fsync performance, and a disk format extension that allows more hard-links inside a single directory.
Hole punching in file-systems comes down to marking a portion of a file as being unneeded and the associated storage to that file portion can then be relieved. Hole punching was previously added to the Linux kernel as a standard interface and has been implemented by XFS and others, while now Btrfs can too punch a hole in a file.
The improved fsync performance is also very significant. Chris Mason's "turbo charge fsync" patch for Btrfs indicates some mighty impressive performance improvements when using the file syncing operation extensively.
For increasing the single directory hard-link limit, a btrfs-progs patch is currently needed to enable the compat bit for the feature.
Chris Mason is also working on some new Btrfs RAID code, but it's not part of this current Btrfs pull request for the Linux 3.7 merge window.
The 3.7 Btrfs pull request can be found on the kernel mailing list. New Btrfs file-system benchmarks against XFS, EXT4, and even Reiser4 are forthcoming on Phoronix.