I tried using Force of Nature on an LFR Horridon the other day, just for the sake of experimentation. I figured that since you can meter them out now, they would provide some situational utility by taunting and tanking Warbears briefly. I was really disappointed - they're terrible. Zilch for threat production and even if they do hold threat, they can't take more than one hit before they fall over. Maybe they could be used when soloing, but most bears just round up as many mobs as they can and Thrash spam until everything eventually dies.
How would you go about making those little treants more useful for a raid tank? I think the best thing might be to remove the taunt, but make their melee hits generate Rage for you. With that simple change, Blizzard could make them a legitimate option for tanks.