BtrFS is still far behind ZFS in terms of both features and stability, but I really hope that it will catch up one day. When it supports Raid-Z/Z2, file/block deduplication and having a fast SSD LFU-cache that always holds the most used blocks, I will switch in an instant.
Dedupe is being talked about as far as I know and is a goal of the project,
and it has optimized mode for working with SSD. So on and so forth.
Btrfs is still about a year away, I figure. And when you make a major change
to a project, like going from FreeBSD to Debian for CoreNAS stuff, it's
going to take at a minimum several months to create a working product and
that is why I figure that it's likely that by the time CoreNAS is out the
door Btrfs will be in a usable state.