I really hope this ability is reworked by release. Currently this ability for druids and rangers sets the characters AC to 16. I believe (but could be incorrect) that even if the target has a higher AC it will lower it to 16 if you cast barkskin. So in most cases characters would be lowering their own armor value as 16 AC is pretty easy to exceed for both druids (assuming human shape) and especially rangers. On top of that the spell requires concentration. So if you cast barkskin you are burning a lvl 2 spell slot to set your armor class to a likely lower value and disabling your ability to use any other concentration ability (like hunters mark or entangle). I wouldn't mind the cost of concentration if the trade off was worthwhile, unfortunately it's nerf at the moment.
The spell would actually be worth picking if it either added a set armor value to our existing AC or gave some amount of temporary hit points. I would personally prefer adding AC mainly because temporary hit points do not stack and can be acquired from a few other sources. I would feel better about temporary hitpoints if you could refill or stack them (stack them within reason of course).
