The Btrfs file-system has various "shiny" features like support for copy-on-write snapshots, cloning, transactions, sub-volumes, SSD optimizations, transparent LZO/Zlib compression, and many other advanced features by Linux file-system standards. The problem with Btrfs is that the next-generation file-system is still in development and hasn't yet been proven via years of use and testing yet as being a stable and reliable solution. Most Linux distributions continue using the EXT4 file-system by default, but now there may be snapshot support coming back to EXT4...
Just use ZFS. Especially for NAS. No need to wait.
ZFS is proven for more then 5 years in production with several hundred TB and tons of disks.
It is supported under Solaris, OpenIndiana, OpenSolaris, Schillix, Nexenta, FreeBSD, Zfs-FUSE and There is working native Linux kernel module in testing and development.