Cast Thorns on tanks to help them keep aggro on multiple enemies. Always keep Thorns up on yourself when you are tanking to provide some additional threat.
Although this spell doesn't sound too useful, it doesn't cost much mana, and is instant cast. Over the ten minutes that the spell lasts, the damage dealt will add up. In general, try to always have it active while solo.
You generally should not cast this on casters while in a group, as the damage caused by Thorns does increase aggro, making it harder for others to pull mobs off them.
Thorns will act as a buffer against dispels in PvP, giving enemies a chance to dispel your Thorns instead of some more important buff.
The damage caused by Thorns can, due to lag, sometimes break crowd control effects like Hibernate and Polymorph. Because of this, if you or someone in your party has latency of over 500, you may want to avoid using Thorns if you are using any in-combat crowd control.
Thorns can be very useful when being attacked by an enemy that has very low health as they will often die quickly from the Thorns damage without requiring your attention. Many creatures summon minions that have only one hit point - Thorns will effectively dispatch these enemies for you.
As Thorns benefits from your spell power at the time of casting, you can activate any trinkets that increase spell power and equip items with high spell power before casting it to maximize the effect. This is especially useful for feral druids with the Glyph of Thorns, as they can equip their Balance or Restoration gear before casting the spell, and will then receive the benefit for a full hour before needing to recast it.
Refreshing and Spell Power
When refreshing Thorns on yourself, you may sometimes get the error message, 'A more powerful spell is already active.' This is understandable if the initial buff was cast by a different druid with a higher rank of the spell or talents to increase its potency, but may be confusing when it was you who cast the spell. The reason for this error message is that Thorns benefits from the druid's spell power at the time it was cast, not at the time of the hit. Thus, casting Thorns on yourself while at a higher spell damage (such as with a trinket activated) will make it impossible to refresh the buff without first right-clicking it off if your spell power has dropped down. Some buffs and items may temporarily increase spell power, causing this to occur. If using similar items or abilities with short cooldowns, it would be best to max out spell power before casting Thorns to get the maximum effect out of it.
Hotfix (2009-05-22): "The bonus damage from spell power on the Druid ability Thorns has been reduced by half."