The thing I miss most in ZFS (aside from a native in-kernel linux port, as I've found ZoL to be kinda buggy) is to be able to grow zdevs. I can add a new zdev, I can remove them, but I can't add one disk to my 7-disk RAIDZ2. MD RAID has no problem with this.

Also, if you add zdevs later, you will never get full performance again as the striping will be off (putting more writes on the zdevs with more free space), and there's no 'rebalance' kind of thing (I've seen some scripts that just move stuff around, but I've never seen any evidence that they work, and I really doubt that they do based on my understanding of zdev allocation).