Shadow Priest

I don't know or recall all the numbers and theory crafting behind all of this, but I have played my spriest for about 2 years now and I try and stay current with her through Elitist Jerks, Icy Veins, and some of the WoW forums. This is what I have:

Stat Importance: Int>Spirit/Hit capped> Haste> Crit / Mastery
Spirit is the same as hit for Shadow Priests, so always choose Spirit over hit since spirit regens mana.
Also - I don't mind sacrificing a little hit if I can throw it into haste and get over a threshold. I never sacrifice more than would put me more than 1% miss chance on a 93/boss though.

Buffs: Shadow Form, Fortitude and Inner Fire (always up)

I use Shadow Priest Dot Timer addon to help me keep an eye on dots.

Single Target Rotation:
SW:Pain (always keep this dot rolling on enemy)
Shadowfiend (always keep this on cool down)
Mind Blast (always keep this on cool down)
Vampiric Touch (always keep this dot rolling on enemy)
PW:Shield (always keep this on cool down) (see Glyph notes)
Mind Flay until Mind Blast comes up. (MB generates a shadow orb)
Mind Flay until Mind Blast comes up. (MB generates a shadow orb)
Now you have 3 shadow orbs and your Devouring Plague is lit up.
Hit Devouring Plague (see Solace and Insanity talent below)
Mind Flay over and over until it is time to refresh dots and buffs
SW:Death when it lights up. (generates a shadow orb) can only be used when enemy is at 20% health or less. You can cast it twice in a row but it will only generate an orb on the 1st cast.

Halo - (talent) Be aware of your surroundings when using this. This can pull mobs if they are nearby. I try to keep this on cooldown as much as possible in raids and dungeons. It heals allies and damages enemies. Keep in mind, it does more healing and dps to targets that are farther away from you.

Dispersion - use when you need mana or to mitigate damage.

Vampiric Embrace - Great to use when healing is needed, individually or in a group. It heals for 100% of any SINGLE TARGET shadow damage you deal. Its great to cast right after you hit Devouring Plague then Mind Flay.

Multi target AOE:
Throw your SW:Pain up on as many targets as you can
Shadowfiend
Mind Sear (you can also target the tank or any player who is in the middle of the mob and cast this)
Halo (if you can avoid pulling extra mobs)
Be sure to keep your PW:Shield up if there is any damage going out to you.

Talents:
Void Tendrils - Instant cast - roots 5 targets for 20 sec
Body & Soul - Gives speed increase when hit with PW:Shield
Solace & Insanity - Devouring Plague (consumes all shadow orbs) increases the damage Mind Flay does by 33% for each shadow orb consumed.
Angelic Bulwark - Passive shield when you go below 30% health
Divine Insight - SW:Pain dot has 5% chance to reset CD on Mind Blast, make it instant cast and cost no mana. (procs often)
Halo - ring of energy, grows in power as it spreads. Greatest effect at 25 yards

Major Glyphs:
Mind Blast - your crits make the target unable to move for 4 sec
Reflective Shield - 45% of all damage is reflected back to the enemy from your PW:Shield
Vampiric Embraces - Increases Vampiric Embrace's healing effect by 50%

Minor glyphs are all for show or play.