Disclaimer: I only raid 10man, things might be different in 25man raids. Also, I don't know everything.
In almost every post in this blog I've been going on about my +spell hit rating. So what's the big deal really? Why is it so important? What does it do for a shadow priest?
First of all, let's look at the concept of spell hit. Wowwiki says: "The more spell hit gear a caster has, the lesser the chance an enemy will resist his spells." As the game is set up there will always be 1% chance to miss, no matter what we do. It's the remaining percentage that we're concerned about.
If you throw a spell at a mob that's 3 levels below you, your hit chance is 99%. If you throw a spell at a mob that's the same level as you, it's 96%. When you get into Karazhan and throw a spell at a mob that's 3 levels above you (or any boss), your chance to hit is 83%. That's getting a little low. Since it's obviously necessary to down mobs and especially bosses as fast as possible, it's vital that almost 2 out of every 10 spells you cast are not resisted. Any spell that is resisted is a waste of casting time and mana.
Enter +spell hit.
PvE raiding mobs marked with a skull are considered as level 73 and thus you have 83% chance to hit them. At level 70, you gain 1% hit for every 12.6 spell hit points you've gathered from gear. The theorycrafters have done the math for us and tells us that 202 spell hit rating converts to aprocimately 16% and 83 + 16 = 99. In summary: to not resist more than the unavoidable 1%, a shadow priest needs +202 spell hit. That's a lot.
Enter Shadow Focus.
Shadow Focus is a tier 2 talent in the shadow tree and has a maximum of 5 points. For each point in this talent, you gain 2% spell hit for all shadow spells. Since pretty much everything we do is shadow magic, that's one hell of a useful talent! In fact, it's imperative for a raiding shadow priest to max out this talent until they get their +spell hit going.
(The talent Focused Power also helps the +spell hit for Smite, Mind Blast and Mass Dispel, but being in tier 6 of the Discipline tree it's really not worth it IMO)
So let's say you're level 70 and have no +spell hit but you just specced 5/5 in Shadow Focus. You now have an extra 10% chance to hit with all your shadow spells, landing you at a comfortable 93%. You will need 76 hit rating in order to cover the remaining 6%, which is a much more reasonable number. Once you reach 101 hit rating, you've covered 8% and can downspec to 4/5 Shadow Focus and spend that point on something else. The full list has been posted before but I'll throw it in again:
76 hit is the cap with 5/5 Shadow Focus
101 hit before you can go to 4/5 Shadow Focus
126 hit before you can go to 3/5 Shadow Focus
152 hit before you can go to 2/5 Shadow Focus
177 hit before you can go to 1/5 Shadow Focus
202 hit before you can go to 0/5 Shadow Focus
All numbers rounded up to the next whole number.
When I digned 70 I had a puny amount of +spell hit because I didn't know what on earth it was. Once I figured it out with a little help from my guildies I set about gathering any +spell hit piece that was available to me. For example the day I realised that 2 pieces of the Mana-Etched set gave +35 spell hit was a glorious day and it got a big ol' post on this blog. (I'm still using those.) It's not easy accumulating all that hit but it's not easy to down bosses without it either, and most high-end guilds will require you to have 202 spell hit as one of your most prioritized goals. I won't say the most prioritized because guilds differ, obviously.
That said, it's not only gear that can help you hoard spell hit. Xornot's Gem Finder, which is linked to the right, lists all relevant gems at the click of a button. Any enchanter that's revered with Cenarion Expedition will be able to put +15 spell hit on your gloves. (Consider getting that instead of the +20 spell damage enchant until you've reached your cap.) There might be other ways too, but none that I'm aware of.