It only debuffs defense. It debuffs resistance to all the others and adds resistance to healing. See below for CoD (these are defender numbers).
-25% Base Defense (all affected targets) for 45s
-40% Resistance (Endurance, ToHit, Regeneration, Recovery, RechargeTime, EnduranceDiscount) (all affected targets) for 45s
+40% Resistance (Heal Dmg) (all affected targets) for 45s
Ice Arrow also debuffs the strength of the foe's secondary power effects (and ignores resistance):
-67.05% Heal Dmg, Absorb, Endurance, RunningSpeed, FlyingSpeed, Confused, Terrorized, Held, Immobilized, Stunned, Sleep Strength (all affected targets) for 60s
-67.05% Ranged, Melee, Area, Smashing, Lethal, Fire, Cold, Energy, Negative Energy, Psionic, Toxic, Base Defense, ToHit Strength (all affected targets) for 60s