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
Most features are done and you can install with btrfs out of the box today in the RHEL 6 or Centos 6 betas.
http://en.wikipedia.org/wiki/Btrfs