Originally Posted by
lampy.
There are two stats for priest spells: hit rating and spell penetration.
Hit rating means the spell hits or it misses. There is a 4% chance to miss vs a lvl 80, not counting racials. The blood elf racial gives them an additional 2% chance to not be hit by a spell. Most priests go for 6% hit rating, or 158 hit rating.
Spell penetration deals with resists and damage reduction from spell resistance. Shadow Protection, Shadow Resist Aura, Mage Armor, and Mark of the Wild all provide resistance vs shadow spells (Fear, Dots, Mana Burn, etc.) Players can resist a fear, in addition to a fear missing. The golden number for spell penetration is 130 because it takes care of everyone except a Mage with mage armor + another shadow resist buff.
Back enchant - 35 Spell Pen, Furious Glad Wand - 31, Off-hand - 55 gets you close, so you'll need to use a few gems to hit 130.
All Disc spells count as Holy spells and as such are not affected by resists, so they will never be "resisted" but can still "miss". You will get a ton of misses when trying to mass dispel Ice Blocks and Pally's Bubble because of those classes talents preventing their buffs from being dispelled by an additional 30%.
As a disc priest in arenas, you will heal, fear (CC), dispel/mass dispel, and help DPS the target when able. In order to do these things well, you'll need the 158 hit rating and the 130 spell penetration.