Among the Btrfs Linux 3.4 changes is the merging of error handling patches from SUSE, reworked mata-data with page cache interaction (performance improvements!), more aggressive page dropping for freed meta-data blocks, and support for meta-data bigger than the page size (meta-data blocks up to 64KB in size while 16KB / 32KB sizes seem to work the best). There's also other fixes and updates, including to the Btrfs balancing and defragging code.
The SUSE patches have already been part of the SUSE Linux Enterprise Server kernel and make it possible to abort transactions and to go in a read-only mode upon hitting errors.
Unfortunately, the LZ4 compression and Snappy compression support for greater transparent file-system compression don't appear to be part of this 3.4 merge (though LZO/Zlib compression for Btrfs remains a mount option). Chris Mason also mentions the Btrfs RAID5/RAID6 support is on the table for the Linux 3.5 kernel.
Find out more information on Btrfs in the Linux 3.4 kernel via Mason's pull request.