The story of Thorns:
In the beginning, Thorns was a mediocre 10-minute buff. Its only real use was to aggro you to Booty Bay guards when someone tried to gank you.
Thorns was then buffed to 30 minutes, still doing mediocre damage (akin to Retribution Aura). Balance Druids were able to talent into it, making it more powerful.
Blizzard then revamped the spell, making it a short duration cooldown that did MASSIVE damage. It was also enhanced greatly by spell power.
Finally, Thorns, like most other stuff, was destroyed by the whiners and was turned into the bland and mildly useless ability it is now. Only reason to pop it is if you're planning on soloing a group of mobs, or if you know you're about to be jumped by a melee player.
The main issue is that it shifts you out of your current Feral form when you use it, making it useless for tanking anything that'll gib you if you're out of Bear Form, and greatly reducing its effectiveness in PvP.
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Please "talk up" if your comprehension white-shifts. I will use simple-happy language-words to help you understand.