Frost Armor is a passive level 56 Frostmage ability which increases the mage's haste and has a chance to give attackers the Chilled debuff, reducing their movement speed.
The snare effect is useful for avoiding fighting aggressive mobs you would rather run past, assuming of course they don't resist each time they attack. On foot, a chasing creature will likely get in 2 or 3 hits on the mage (some creatures run faster and swing faster than others) before you outpace them. Mounted characters will usually be hit a single time before getting away.
Now a passive effect and is available only to Frost mages.
Now grants an 8% chance to Multistrike instead of 7% Haste.
Hotfix (2012-09-18):Frost Armor now increases spell haste by 7% (up from 5%).
Patch 5.0.4 (2012-08-28): Frost Armor has been redesigned. Now increases spell haste by 5%. Frost Armor no longer reduces physical damage taken and the Chilled proc does not reduce attack speed. In addition, there is now a 3-second cast time. Frost Armor no longer has a duration and will persist through death.
Patch 4.0.1 (2010-10-12): Frost and [Ice Armor] merged, now available at level 54, up from 1.
Patch 3.3.0 (2009-12-08): The buff will be stored in a Consolidation Box when received. It will come out when 3 min. is left.
Patch 2.4.3 (2008-07-15): No longer a magic effect and cannot be dispelled.
Patch 1.12.0 (2006-08-22): Chilled effect: Due to the haste effect change, the tooltip has been changed to 25%.
Patch 1.8.0 (2005-10-10): Fixed a bug where the Chilled effect caused by Ice Armor/Frost Armor was not removed properly when shapeshifting.
Patch 1.6.0 (2005-07-12): Targets that are immune to movement slowing effects will no longer be affected by the Chill's movement slowing effect. Targets will still have their melee attack speed reduced.