It's completely different in 4.0. Now thorns is of short duration and is on a cooldown. If you want to keep it up, you have to keep recasting it. In exchange for needing such active maintenance, though, it now does gigantic damage output! In 5-mans, I am seeing it provide 20-35% of the tank's overall output, and typically it is the highest individual ability in a damage breakdown. I've also tried it in soloing, and I've frequently seen it do more than 50% of my output, even on fights where starfall and treants were available. Consider that carefully: it does more damage than hurricane, but you don't get locked into channeling.
In short, cast thorns. Cast it early in the fight to give the tanks some initial threat, and then cast it again whenever you can spare the GCD. Furthermore, the Glyph of Thorns is quite helpful, if you don't know what other major glyph to take. I'm using it on both resto and moonkin specs right now.
Now that the spell is so much more useful, I have two questions about it?
- What happens if multiple druids cast thorns on the same tank? Will they both take effect, or just one of them? I'm guessing just one, but a tree can dream.
- Will Skada, World of Logs, and other log parsers ever account the damage back to the druid? Currently you contribute a lot of dps to insert invocations of thorns, but the meters don't give you credit for it.