I'd like some clarification if anyone here is on the PTR:
1. The spell has 3 charges. Does it also have a cooldown in addition to those charges?
Since it has charges, it might/should work like Savage Defense for a Guardian druid. With a Guardian, SD has 3 charges, each lasts 6 seconds and each charge has a 9 second CD. This means that a Guardian can stagger his SDs so that he can do several consecutive SDs without delay or he can spam all three charges at once and then has to wait 9 seconds before he can use any more. (The latter is obviously not an ideal way to use it.) With proper staggering, a Guardian druid could maintain SD for 45 seconds (iirc) before the recharges become an issue. After that, he can maintain a 67% uptime on SD.
If Force of Nature works the same way, but in this case with a 15 second uptime and a 20 second recharge, then that means Resto druids could, with proper staggering, maintain a treant companion for a very long time during a fight before the recharges become an issue. Alternatively, the Resto druid could use all three charges at once every 20 seconds and get the effect of all three 75% of the fight. (And that's a pretty good uptime!)
However, none of this is possible if, in addition to having charges, Force of Nature also has a cooldown. In other words, if you have say a minute to use 3 treants and then the spell goes on CD for another minute or two, then everything I've said above doesn't apply. I doubt this is how FoN works, but I'd still like confirmation.
2. It's been mentioned already, but just to be clear: Are you given the option of either casting FoN *on* another player or else *at* a location? Can I cast FoN on someone the same way I'd cast Rejuv on them? And if I don't have a target when casting FoN, does a targeting circle appear on my mouse?
3. A followup from question 2: It seems, from what I'm reading, that if FoN is cast *on* someone, that they will instantly receive the Swiftmend. If instead, however, I cast FoN at a location, do the treants then cast Swiftmend on whomever they feel like (perhaps choosing the lowest health targets)? And if so, do they cast it as soon as they spawn?