This is an all-around build for dungeon crawling and questing; tested so far up to level 41, now at 4/25/3. I've purposely tried to avoid talents which do not increase "combat power" -- the total amount of damage and healing you do against an implacable foe before you die. Exclusions include talents which solely increase mana or mana regeneration and reduce threat. The build's focus is not so much leveling quickly but having the greatest chance of winning each fight.
Particularly Valuable Talents
My current Spirit is 171, which Spiritual Guidance turns into a +42 to damage and heals before coefficient modifiers. Smite is the Holy tree's "bread and butter" spell (i.e. the best non-DoT DPS spell that has no cooldown), which gains a +30 from this talent after the coefficient modifier -- a solid 13% damage increase.
Three points in Blessed Recovery makes 25% of all melee/ranged critical hit damage go away, if you can survive for the next six seconds -- turning a double damage hit into merely 150% damage taken, just like a spell crit. Well worth taking.
Also nice is Spell Warding -- attenuating all incoming spell damage, and plenty of baddies cast spells and otherwise hit you with elemental damage. (This talent is one of the very few defenses I'm aware of that reduces Holy damage.)
Holy Nova has a high mana cost but gives one another AOE option, does both damage and healing at the same time, and it generates very close to no threat.
On the Discipline side Unbreakable Will, Improved Inner Fire, Improved Power Word: Fortitude, and Improved Power Word: Shield provide much-needed defense for yourself and your group as a whole. By default as "clothies" the Priest class is weighted heavily toward the offense so more defense is very welcome.
At maximum spell level Divine Spirit increases Spirit by 40. Combined with Spiritual Guidance this gives a flat pre-coefficient +10 to damage and healing . . . not bad for only four points invested, and +15% mana regeneration during combat is a nice bonus.
(In my research I was surprised to discover that Spell Power also increases the strength of Power Word: Shield, and the Spell Power from Spiritual Guidance and Divine Spirit is no exception.)
The single Shadow talent to take is Spirit Tap, which when it activates doubles the bonus from Spiritual Guidance for 15 seconds -- you get another +30 and +10 damage/healing from the examples above. This works best while solo questing but can also be a nice occasional benefit in groups -- especially since you're one of the heavy hitters. And Spirit Tap's extra mana regeneration doesn't hurt either.
The five stats you're interested in are elemental resistances, Stamina, bonuses to spell damage, Spirit, and Intellect, in that order. Stamina lets you stay in the fight longer so you do more damage and have more player thinking time to pop potions, heal yourself, etc. to stay alive. Spirit is for increasing the Spiritual Guidance bonuses (and, incidentally, mana recovery). Spell Power simply provides more damage and healing. Intellect is for increasing critical hit chance (and giving you more mana to convert into damage). And elemental resists reduce the chance that spells and spell effects damage or otherwise affect you.
I did a combat power study of my current character and calculated relative values for these five stats, scaled to make a single point of Stamina worth exactly one point . . .
* Stamina: 1
* Elemental Resistance, +1 all: 1.19
* Spell Power: 0.871
* Spirit: 0.218
* Intellect: 0.0186
We can see here that each point of Elemental Resistance is worth 119% as much as a point of Stamina, and so on.
Example: An "of the Eagle" item has +9 Stamina and +8 Intellect; it's value is 9 x 1 + 8 x 0.0186 = 9.149 -- worth a bit more than 9 Stamina, or 91 Health. Another item gives just +10 Stamina, which gives it a value of 10 x 1 = 10. The second item gives more combat power so choose it over the first item.
I wanted to test the extreme case -- wearing nothing but Stamina gear -- so . . . I got naked. I removed all my gear but still had 72% of my normal health and 61% of my normal mana -- a character's base stats are good enough to avoid becoming a total glass cannon with hardly any mana for spellcasting, so equipping as much Spell Power as you possibly can is viable, followed by Stamina. (Stamina has a higher per-point value than Spell Power, but items at an equivalent level tend to have about a 50% higher numeric Spell Power score than Stamina, so Spell Power items usually win.)
Practically speaking if you're lacking reasonable Rares you want to shop for these Uncommon items (assuming near-equal minimum levels) in the following priority (a general rule; doing the math yields more accurate results):
* Elemental Resistance items (which give that bonus to all resists; prorate for items which affect just one or two resists)
* Spell Power items which increase Stamina as well
* Spell Power items
* "of Stamina" items
* "of the Whale" items
* "of the Eagle" items
* Spell Power items
* "of Spirit" items
* "of Intellect" items
(It's rather ironic that Spirit from gear isn't worth very much at all for even a Spirit build, but the numbers don't lie.)
Choosing enchantments generally follows the same priority.
(I've seen at least one claim that +spell damage gear does not add to Holy spells but playtesting reveals this to be simply untrue. I made 200 attacks against low-level mobs and compared the average damage against another 200 attacks while not wearing my Spell Power gear, and there was a definite advantage to wearing the Spell Power gear that closely matched after-coefficient expectations. Maybe no bonus is true in Retail, but not in Classic.)
I try to avoid accepting the healer role in groups -- the build works better for making casters explode, Holy Nova'ing multiple targets, and off-healing in a pinch. (And groups tend to treat healers like slave labor -- to most-all Warrior and Druid tanks it's far more important to not waste rage than to wait for the healer to drink and get back up to full mana, so healers can feel like they're being jerked along too fast for comfort.) But being the main healer is certainly viable, and with little or no gear change one can instantly switch to the healer role if your primary healer dies or has to leave the group.
Food that increases Stamina and Spirit is also very good -- for the extra Health, not so much for the Spirit. (And if you really want to push the envelope, for any set piece boss battle (one where you can stand in more or less one area without having to move around much), just before the pull build a campfire nearby for that extra +4 Spirit and 1-2 more damage!)
When playing solo my usual opener is Power Word: Shield > Holy Fire > Shadow Word: Pain > Mind Blast > Smite x4, then repeating by going back to Mind Blast. For additional targets after the first use the above but skip Holy Fire, which is less time and mana efficient than Smite, and skip SW: P too if a target's health is maybe two-thirds or less. (Shadow spells seem to bounce so frequently -- at least in dungeons -- that I'm considering removing them from my rotation.)
Generally in groups I skip Holy Fire altogether. I hammer targets in target marker order (skull, etc.).
Against casters I generally disregard any target markers except crowd control ones like moon (i.e. Polymorph). I select one caster -- preferably one at full health -- and try to burn him down using the above rotation but swapping SW: P and Holy Fire. As soon as he targets me I pop Feedback (Human Priest racial ability). I have this, plus PW: S, Spell Warding, and some elemental resistances from gear (currently Elemental Rainment, +3 All Resists on my cloak, and +5 Shadow Resist on my wand, plus +30 Shadow Resist from Shadow Protection), so I'm better at tanking casters than the tank, and it takes some of the heat off him. I can typically solo-kill a caster pretty quickly while losing only a third to a fourth of my health.
In the absence of casters, if there's four or more targets relatively close to one another I wait a few seconds to let the tank grab more than zero aggro and let him take a few hits, then rush forward and spam Holy Nova until out of mana or the number of targets becomes less than four, then back off and wand. Besides damaging multiple enemies I'm also healing the tank, myself, and most-all melee group members, including all melee pets like voidwalkers and hunter pets (which are currently problematic to heal because they don't appear on the group list underneath their owners). While wanding I'm essentially a flexible tactical reserve -- I mostly stare at the group's health bars and toss in a few heals as needed, or go to Mind Blast > Smite x4 if my mana regenerates up to about half.
(FOUR is the damage efficiency breakpoint number for Holy Nova -- AOE four or more targets, otherwise attack single targets. Also, Holy Nova works great in a group but poorly playing solo -- you'll run out of mana before you kill anyone, unless you're farming greys that are low enough.)
No casters and less than four targets? I follow target markers and/or burn down the enemy with the lowest Health.
The build compares favorably with Shadow-spec builds, even taking Vampiric Embrace and Shadowform into account; offering an estimated 35% more combat power. Mind Flay is Shadow spec's bread and butter spell, which does significantly less damage than Smite (about a 5:7 ratio), and Shadowform makes Holy Fire -- your best opener -- effectively unavailable. Also, Holy attacks are less easily resisted since there doesn't seem to be such a thing as Holy Resistance -- my Shadow spells seem to be resisted a lot more than my Holy spells (but Shadow Focus can make this less of a problem).
Overall it's a fun, flexible, and otherwise powerful build to play.