Let's see...
RAIDZ is like RAID 5 but integrated into the filesystem (reducing overhead (although ZFS is rather notorious for being a RAM hog for other reasons), increasing safety (no write hole), and reducing rebuild time of a degraded array if the pool wasn't entirely full)
The storage pool system makes for much greater flexibility in how the filesystem is managed (I want a new filesystem that's compressed and with certain security options enabled? Just make it, no partitioning required, and set a mountpoint) Yes, I know, Linux does have LVM, but it's nowhere near as flexible, and requires much more planning ahead. (Although, ZFS on FreeBSD requires more planning than it does on Solaris - a boot partition and a swap partition have to be outside of the ZFS pool (you COULD put swap inside the ZFS pool, but there's apparently a race condition in the FreeBSD kernel, that means that swapping to ZFS can freeze the system. Still, a tiny boot partition with the ZFS bootloader, and a swap partition, isn't that much outside of ZFS.)
Snapshot support is nothing short of excellent (major system change? Take a snapshot of your root filesystem beforehand, and then if the shit hit the fan, just boot from the snapshot). I don't think you can do that with a LVM snapshot?
Here's what I've got now:
root@uncannyvalley:/home/bhtooefr # zfs list
NAME USED AVAIL REFER MOUNTPOINT
zroot 36.4G 1.69T 192K /zroot
zroot/ROOT 383M 1.69T 383M /
zroot/home 13.2G 1.69T 13.2G /home
zroot/srv 18.1G 1.69T 2.76G /srv
zroot/srv/torrent 597K 1.69T 213K /srv/torrent
zroot/srv/torrent/downloads 192K 1.69T 192K /srv/torrent/downloads
zroot/srv/torrent/watch 192K 1.69T 192K /srv/torrent/watch
zroot/srv/www 15.3G 1.69T 15.3G /srv/www
zroot/tmp 485K 1.69T 485K /tmp
zroot/usr 4.15G 1.69T 366M /usr
zroot/usr/local 975M 1.69T 975M /usr/local
zroot/usr/obj 192K 1.69T 192K /usr/obj
zroot/usr/ports 2.24G 1.69T 1.86G /usr/ports
zroot/usr/ports/distfiles 387M 1.69T 387M /usr/ports/distfiles
zroot/usr/ports/packages 202K 1.69T 202K /usr/ports/packages
zroot/usr/src 615M 1.69T 615M /usr/src
zroot/var 568M 1.69T 3.43M /var
zroot/var/crash 197K 1.69T 197K /var/crash
zroot/var/db 560M 1.69T 551M /var/db
zroot/var/db/pkg 9.15M 1.69T 9.15M /var/db/pkg
zroot/var/empty 192K 1.69T 192K /var/empty
zroot/var/log 3.08M 1.69T 3.08M /var/log
zroot/var/mail 202K 1.69T 202K /var/mail
zroot/var/run 464K 1.69T 464K /var/run
zroot/var/tmp 202K 1.69T 202K /var/tmp
You might notice that they all have the same available space - they're sharing the same storage, and grow and shrink on demand. I could constrain any of them as needed, but don't see the need right now. (The torrent directories aren't populated, mind you.) Some of them (/var/log, for instance) are compressed at the filesystem level, and it's almost as easy to make a filesystem on ZFS with custom attributes as it is to make a directory.