Reduces the chance your spells will be dispelled by X% and reduces the threat caused by your Arcane spells by Y%.
Arcane Subtlety is a Mage talent that grants Dispel resistance while reducing the threat caused by Arcane spells. It is a very useful talent for arcane mages across both PvE and PvP as it offer multiple benefits simultaneously.
Dispel resistance is invaluable in PvP situations, specially for a class as reliant on buffs as the Mage. It can prevent both your buffs and the dots you place from being dispelled, protecting from Purge, Dispel, Mass Dispel, Spellsteal and others of the sort.
The threat reduction is excellent for mages that use Arcane spells as a primary damage source in raids. 40% threat reduction to Arcane spells is the most that a Mage can obtain through talents, with Frost and Fire talents reducing threat of those schools by only 10%.