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(1) ADDONS: Download either Grid+Clique (two separate addons) or Healbot. Yes, I know some folks get the job done w/ mouseover macros and whatnot, but I have yet to find anyone in-game who hasn't seen their healing efficiency greatly improve by using one of these. If you decide to go holy, I suggest downloading PowerAuras as well to better track your SoL procs.
(2) DISCO: Some folks have a few points in other places, but in general, a good spec is:
57/14/0
(a) SPELLS&ROLE: As with any healing, there is no "rotation." The spells you will use most frequently in Disco are: PW:S, Penance, Flash Heal, ProM, BH, & PS. As a Disco priest, your main function will be tank healing.
PW:S: PW:S should always be cast on the tanks when the WS debuff wears off on them. Even though your primary duty is to keep the tank alive, you should take advantage of your ability to throw PW:S on the raid. If the tank is topped, feel free to put PW:S on a raid member who is taking damage, or on key persons in your raid. Not only will this help the raid healers, but it will increase your mana efficiency through Rapture.
ProM: Should always be cast on the tank on CD.
Penance: As of right now, Penance is usually casted on CD on the tank(s), however, with the upcoming CD nerf in 3.2, it would be wise to practice saving this spell for when the tank needs Burst Healing Now(tm).
Binding Heal: BH should always be used when you need a heal. If the tank is topped off, cast this on a raid member who needs healing.
Pain Suppression: PS is your DR CD. It is used for when the tank is taking a known massive hit that would else normally kill him. Think of it as an extra shield wall for your tank.
Flash Heal: Flash Heal is your "filler" spell. This should be used on the tank/raid when the WS debuff is still active and ProM and Penance are on CD.
(b) GLYPHS: Penance, FH, and PW:S are the most commonly used glyphs as Disco. Minors should be SF, Fade, and Levitate. There are other useful glyphs, but these are the standard.
(c) GEAR&STATS: Priority for gear should be SP>INT>Mp5>SPI. For enchants use what makes sense. Gem for INT.
Haste vs. Crit: These stats should be balanced for maximum effectiveness. Haste makes your FH & Penance faster, good for raw through put when the tank is taking massive damage. However, Crit has more synergy with Aegis, a spec defining ability. As a rule, Crit is desired over haste. Neither should be enchanted/gemmed.
(3)SPELLS OF EQUAL USE TO BOTH SPECS:
Shadowfiend&Hymn of Hope: Your main abilities for mana regen. The mechanics are a bit clunky, but if used properly will return a huge chunk of mana:
(1)Cast Shadowfiend
(2)Cast HoH until you receive the 20% mana increase buff.
Using these in concert will bring most priest from near OOM to near full.
Divine Hymn: Used as your raid wide "OH SHIT" button. Macro this to Inner Focus to both negate the massive mana cost and increase each heals' crit by 25%.