I'd also like to point out the importance of key bindings, if I were starting afresh I'd probably go for the spells in order 1-9 which I'd aim to cast on rotation, for historic reasons (sticking spells on the castbar in early levels and getting used to it) I don't have this luxury, even after my year on the side lines I still found my fingers gravitating back to old key bindings.
For example I have 1:MS, 2:MB, 3:MF, 4:SWP, 5:DP, 6:VT, 7:SWD and so on.. now the eagle eyed amongst you will notice that Mind Spike wasn't available till recently, am I a clairvoyant keeping the 1 key unbound for 5 years in anticipation? No, Mind Blast used to be there, but as MS and MB go so well together for trash & questing I decided to shift everything right a step, keeping the same order so my fingers didn't have to do too much thinking. I also have a universal PANIC button which for me is the ' key (the one to the left of the 1 key), I know, whichever toon I'm playing, if bad stuff is happening this is the key to go to, for my Shadow Priest it's dispersion, spec'd holy it's Guardian Spirit (did I mention I bit the bullet and sorted my offspec out?) on my mage it's Ice Block etc. it's usually hit when I've done something silly and am about to die. (over aggroing, or not moving out of the crap on the floor in most instances)
On that note, a word on which spells to use, for trash, adds or anything else which goes now before I can get a full Vampiric Touch off (i.e. cast it on the mob and it ticks till it ends) I go MS MS,MB,MS,MS,MS,MB (3x MS after the first MB cast as MB will still be on cooldown), throwing in the odd SWD for good measure; if it's not dead by then simply rinse and repeat (actually you probably want to consider the normal DPS rotation if it's not dead by then).
For longer lived mobs and bosses, I tend to go SWP, DP, VT, MF, MF, MB, VT, MF, DP, MF…. And so on to keep dots refreshed. Now a few things of note here, I only use MB when I have at least one shadow orb, and only if the Empowered Shadow buff has lapsed (or is about to) else carry on Mind Flaying – note the buff is applied to you, not as a debuff to the mob. Make sure you have SWD glyphed, and make sure you start spamming it, health allowing, when the mob is under 25%, there's no point using it unless you're short on mana otherwise (more on mana later). The skill comes in chosing where and when to throw in the longer cooldown spells, like Dark Archangel, Shadowfiend etc. and how to adjust your rotations to allow for it, more on that later.
You can clip dots now (i.e. overwriting a dot won't stop the final proc damaging the mob) the new dot will simply extend the time, you still want to aim to cast as late in the dots life as possible though else you're wasting time you could be casting something else. Mind Flay still clips, so wait for it to finish / play around with your lag settings to stop this happening, there's a 'nochan' type macro command you can use to stop it all together but I feel it's a bit restrictive
That's about it for DPS in the instance, the real trick to DPS is in the preparation, know what stats make you do more and plan for it, knowing your stats will also make you more able to work out if gear dropped by bosses is better than your current stuff. Need to work out whether a belt with 100 Int and 50 Haste on it is better than your current one with 110 Int and 60 Crit, here's how:
Stat weightings are a normalised figure (that is the 'best' stat is always given the value of 1, so a stat with 0.5 weighting is half as good as the top stat, one with 0.25 is a quarter as good and so on). It is important to note that your stat weightings will change with your gear, so the only way to work out your exact stat weights is to run the theorycraft simulator with your exact gear, and even then your buffs in any given instance will change this. Unless you're obsessive compulsive, or just plain sad, these stats below will suffice (from shadowpriest.com - they're from massive simlecraft tests using t11 gear, they won't be spot on for you but won't differ wildly).
- Int: 1.0000
- SP: 0.7935
- Haste: 0.5031
- Crit: 0.4040
- Mastery: 0.3857
- Hit: 0.3741
- Spirit: 0.3732
This basically means Int is TWICE as important as any other stat (you don't get much with spell power any more) you should look to reforge from Spirit, Hit (if you're hit capped) and Mastery to Haste and Crit and haste where possible. I've seen some pretty confusing information on hit caps since Cata's release, some pages claiming that you don't need to be hit capped, I'm not convinced on this, I've yet to run the numbers but anecdotally I've seemed to do less damage when I've taken hit gear off, I'll prove this one way or the other sometime soon. Mastery sucks donkey balls for shadow priests, I suspect it will come into its own as people climb up the gear ladder or as blizzard issue patches (it's really important for holy priests so if you're doing any healing look to get a second healing outfit), hit is pointless past the 17% cap – extend your character pane to see your values, the nice men at Blizzard now do this calculation for you. Spirit is like a broken pencil, pointless.
I've read some stuff which suggests as your Haste rating improves, Crit becomes more valuable, I'm yet to test or look into this, I'll update this post when I know, but given the new mechanics on clipping this makes sense in theory.
Mana Conservation & Regen
As I mentioned earlier, there are several mechanics for preventing a Shadow Priest running out of mana, to the extent that if you're out of mana, you've done something very, very, wrong (or silly). In order of importance, here's the spells I use to ensure I (and other party members) always have enough of the blue stuff:
Vampric Touch should be up all of the time, it is one of your main sources of DPS and regens mana of both yourself and your party members.
Dark Archangel consumes your dark evangelism stack and gives you 25% of your total mana back, it's also a dps boost on a shortish cooldown, for long fights wait till you're sub 75% on the mana and hit it (so you don't waste mana), for shorter fights, or if you're confident you won't run out of mana, hit it as soon as you get the stack of 5 evangelism
Shadowfiend – obvious – in long fights cast it early so that you can use it twice (make sure you don't hit 100% mana or it's a waste) it also gives a reasonable dps increase so look to use it even if you won't go oom.
Shadow Word Death should only be used in your dps rotation IF you are short of mana or IF the mob Is below 25% and IF using it won't kill you (or cause major problems for the healer) - it's been changed to a execute spell but is also the shadow priest equivalent of life tap – with the masochism talent any damage above 10% of your HP will refund 10% mana – glyph SWD so you get two cracks when the mob is under 25% if it doesn't die. Now as someone who's spent a lot of time healing, there's nothing more annoying than having a tank on 10% health, the party (or raid) shedding health quicker than anorexic on a diet and an idiot Warlock sat in the corner self-harming with life tap, please don't be a Warlocks, think before you click things…
You definitely also want to glyph spirit tap, this gives you 12% mana if your SWD kills a mob, very handy for trash but exceptionally useful in boss fights where there are adds to kill.
Dispersion is excellent for mana regen, but stops your dps, this should be your last resort, and should be saved if the fight is a nasty one where you need to soak up a lot of damage.
If you're really, really, screwed have the hymn of hope somewhere on your cast bar and hit that, I tend not to use it at all unless I've had to come out of shadoo form to help heal, or the healer is running dangerously low on mana.
Enchanting and Gems
Look to enchant and gem in the same way as you'd look for stats for gear, Int first, then Haste, then Crit. Pay attention to the gem slot bonuses, yes a 30 Int gem is better than a 15 Int and 15 Spirit one, but does fitting the correct colour, lesser, gem to a slot give a bonus? Does having that gem in slot activate your meta gem? etc. Be wary of the knock on effects of your choices. I'm not going to link the exact gems or enchants you should go for, have a look on the database at Wowhead.com, I'm intending to go for the cheaper choices until I start getting the better gear, if you're an Auction House legend an have money coming out of your ears then knock yourself out and go for the premium stuff on that ilvl 333 blue…
For more reading, Shadowpriest.com is a good source of info, however it's desperate for a clean-up and is generally difficult to find the right info for the current patch, it's not for the uninitiated and can be very daunting at first due to the volumes of information on there.
This is an excellent guide on elitistjerks http://elitistjerks.com/f77/t112651-shadow_priest_cataclysm/
Hopefully all the information is correct and up to date, I'll look to make corrections if stuff is pointed out to me, and either update this post, or mark it as out of date come future patches.