This information is correct as of patch 5.1a
Unlike melee combat, it seems that spells are on a two roll system for hit and crit. The system rolls once to see if you hit.. and then rolls again to see if you crit. This means that if you can’t hit, you can’t crit.
Your hit rate is a combination of your level vs the mob’s level with hit rating+expertise sprinkled over it. Miss rates are different in PVP.
In the bad old days casters could only ever get to 99% hit rate on spells. There used to always be a 1% chance for the mob to resist. In patch 3.0.2 this 1% always-miss was removed.
There used to be a talent, “Suppression”, at the top of the affliction talent tree which reduced your chance to miss with by 1% per talent point (up to 3/3 for 3%). This was removed in patch 4.0.1.
In patch 4.0.1 the Draenei racial Heroic Presence was changed to give themselves (not their party) 1% spell hit. This is listed here only for the sake of completeness.. since Draenei can’t be warlocks (which is ridiculous if you ask me.. but no one asked).
New, with Mists of Pandaria patch 5.0, spell hit can be obtained from expertise. Effectively we can add together expertise rating and hit rating and use that as our new hit rating to calculate hit percent. So effectively 1% expertise = 1% hit.
Also new with Mists of Pandaria, patch 5.0, each race now has a racial trait which provides passive expertise bonuses when equipping certain types of weapons. This racial bonus translates to a flat 1% hit if you are equipping that weapon type in your main hand.
It is now harder to hit PVE mobs than it ever has been in the past. The new values are below.
At 90, 340 hit rating = 1% hit
At 90, 340 expertise rating = 1% hit
In her skivvies, at level 90 Lilac’s spells will hit a level 90 mob 94% of the time. This means she needs to “make up” 6% of hit. That’s 2040(340*6) hit rating for her to never miss even level mobs.
In her skivvies, at level 90 Lilac’s spells will hit a level 91 mob 91% of the time. This means she needs to “make up” 9% of hit. That’s 3060(340*9) hit rating for her to never miss mobs one level above her.
In her skivvies, at level 90 Lilac’s spells will hit a level 92 mob 88% of the time. This means she needs to “make up” 12% of hit. That’s 4080(340*12) hit rating for her to never miss. Bosses in Mists of Pandaria Heroics are never higher than level 92.
In her skivvies, at level 90 Lilac’s spells will hit a level 93 mob (or skull level boss mob) 85% of the time. This means she needs to “make up” 15% of hit. That’s hit rating for her to never miss 5100(340*15). Bosses in 10- and 25-man raids are never higher than level 93.
In order to hit 100% of the time with all of her spells Lilac will need 5100 hit/expertise rating.
But, because Lilac is a gnome and has a sword equipped, she only needs to make up 14% hit which is 4760 hit/expertise rating.
My understanding is that the prevailing thinking is that hit is less important and you shouldn’t aim to get hit capped. That said, I don’t -like- it… I don’t like the -feel- of missing. I end up at about 5% hit when I follow the SimulationCraft recommended stat weights. That translates to 1 in 10 casts missing. I dislike that so much… you can’t even imagine. I think I dislike that MORE than I dislike “lower dps”.
Keep in mind, I’m also not willing to change my race into an orc, nor am I willing to drop tailoring/herbalism to take up engineering/enchanting (the current FotM favored professions). So as much as the math geeks at SimulationCraft have calculated the “best” stats for DPS.. yeah I’m kind of ignoring that.
So for my happiness, I am aiming to get close to hit cap.. but not going nuts about capping it completely. With AskMrRobot it means I’m using their “PvE: Hit Cap Build” but not forcing it to get to exactly (or over) 15%. It means that occasionally I’m missing.. and it may be that later in the expansion I relax this stance and drop hit in favor of higher DPS.. but honestly.. I don’t think I will. I just hate missing -that- much.