ZFS has the following that BTRFS doesn't
* Deduplication - Huge in any virtualization environment
* Cache layer - Required for high IOPS
* fsck - BTRFS cannot fix itself
* raid - BTRFS raid currently cannot repair itself
and current git of the latest linux kernel still has these isses as well as space issues. I wouldn't trust BTRFS to store /dev/random
Good, because it's an experimental filesystem. Don't write it off completely though, because all of those (except for "cache layer" -- wtf does that even mean?) are planned.
