One of things least understood when it comes to mages (priest and locks too) is what do all the pluses to spell stats mean. There are currently five spell stats in the game (and a sixth only on a few items): Spell Hit, Spell Haste Rating, Spell Critical, Spell Damage, Spell Penetration, and the sixth is spell pushback/interrupt resistance.
Spell Damage speaks for itself. The more you have, the more damage you do. Mages need 500+ spell damage to raid. its not so important in PVP due to resilience lowering all incoming damages and criticals.
Spell hit is the most obvious. All it does is increase your chance to hit the target with your spells, which means it lowers the number of full resists you see. The base chance to resist at even level is 96%, but all raid bosses are set at 73, meaning your chance is at 83%. At level 70 12.6 spell hit adds 1% of chance to hit. You can see why this stat is so important. You need around 200 to cap out your to hit at 70. This stat is important in PVP because it effects your chance to hit no matter what.
Spell Critical increases the chance to critical. The base chance to crit is a complicated formula for each mana class, so I will let Wowwiki explain:
- Every class except Warlocks uses: (Intellect/80) + (Spell Critical Strike Rating/22.08) + Class Specific Constant
- Warlocks use : (Intellect/82) + (Spell Critical Strike Rating/22.08) + Class Specific Constant
- Class Specific Constants:
Now this might seem like a more important stat than spell hit at first, since crit == more damage, but its not, until you get the cap you want to go for to hit. But not for everyone. Since I am a frost mage, I get the same benefit from both, due to my crits doing double damage.
Spell Penetration lowers your targets effective resistance to your spells, which increases your chance to hit them. It is a useful stat, but it does not put a target into negative resistance, meaning it usefulness is limited. It is almost only a PVP stat, very few bosses have real resistances to anything. Even as a pvper you really enough to lower a target’s resistance by 70 to knock most people down to 0 resist.
Spell Pushback/interrupt is an interesting thing. First we have to talk about interrupts. Interrupts/pushbacks (they are the same thing) is what happens when you are cast/channeling a spell and you get hit. As far as I know there is nothing in the game that increase push back and besides talents there are few items that increase resistance to it.
Spell Haste Rating is the new hotness thanks to changes in recent patches. You need 12.5 points to get 1% spell haste bonus. Before 2.4 this meant you could cast fast enough to get hit by the global countdown of 1.5 seconds, but since the patch you can now lower the global cooldown to 1.0 seconds. And it now decrease how long you need to fully channel a spell. That is why I suggest everyone get around 200 spell rating after they max to hit. Especially pvpers.
PvP should focus on Spell hit and Spell haste with minor attention to crit and penetration, and very little in damage. Raiders should do damage, hit, crit haste then penetration. Farmers should go all hit and damage.
That should about cover it.