The Mirror Images inherit the mage's threat lists, and the mage also gains a 90M threat reduction debuff like [Fade]. This makes it useful as a last-ditch threat dump.
Any players targeting the mage when the spell is cast will find themselves targeting one of the images instead, and may waste valuable attacks on the image, or valuable seconds searching for the real mage. This makes it a very useful escape technique in PvP.
Mirror Images often do not attack the highest threat target when summoned and may also attack mobs not engaged in combat, but within range.
When a mob is attacking the mage and the images are summoned, the mob may still continue to attack the mage.
In certain locations, like the room of the third boss in the Stonecore, summoning the images pulls all mobs in the room on the party.
Images will no longer trigger the death sound when their time expires.
The mirror images will no longer complete casts of Frostbolt on targets which are Polymorphed at the time their Frostbolt channel finishes.
Patch 3.1.2 (2009-05-19): The images summoned by this spell will now target the creature that most hates the Mage, and should no longer cast [Fire Blast] or [Frostbolt] on targets that are affected by crowd control debuffs that break immediately on damage unless they are already casting these spells when crowd control is applied.