It's too early for cookie-cutter builds to exist, particularly for disc, which has a lot of options in this expansion. There are some good general builds I can provide, though. First, you should know that there are two very different directions discipline priests can go, and you'll need to choose one. One direction is to take Archangel, Evangelism, and Atonement; the other is to not. Make sure you read over the tooltips for both talents really quick to understand where I'm going from here.
Archangel was previously a talent oriented around mana return; now, it is a talent oriented around throughput. The talents should not be thought of as mana return talents; that aspect of Archangel is only a small perk. Instead, the talents can serve two purposes. The first is to stack up Evangelism through Smite until a 5-stack is acquired, then use Smite as your efficient heal to heal a tank instead of Heal. As long as your tank is the lowest health party member in melee, your Smite will heal him. (The times when you heal someone else because they're lower is something that you can work around using your other spells.) At 5 stacks, the spell is slightly less efficient than Heal, but it's also slightly faster than Heal. Evangelism also reduces the spell cost of Penance, which you'll get to use more of due to the talent Train of Thought, which reduces the cooldown of Penance each time you cast Smite.
The alternative method is to ignore Archangel and Evangelism and instead use Heal as your efficient heal. You'll have more control where you put your heals and you'll be able to use Power Word: Shield more frequently on your main target. This will ensure that you're milking Rapture for what it's worth on your primary target and also increase your Borrowed Time uptime (which works well with Heal's slightly longer cast time.) Skipping the Archangel route frees up 5 talents in the upper tier and will allow you to siphon points into talents like Desperate Prayer and Inner Sanctum for extra survival or Darkness in the shadow tree for extra haste.
Both paths are valid . The amount of healing done by both are almost the same. The difference in efficiency is marginal (though I'm yet to calculate how marginal), because while Penance is a more efficient spell to use more of than Power Word: Shield, reducing your Weakened Soul cooldown means you'll be maximizing your Rapture procs better. Personally, I like to take Archangel and Evangelism and mix it up between using Heal with Strength of Soul, and Smite with Train of Thought.
- Improved Power Word: Shield This buffs your Power Word: Shield and should obviously be taken.
- Twin Disciplines One of disc's core talents for buffing all damage and healing. It's beneficial to every priest, regardless of talent build, and should be taken.
- Mental Agility This talent isn't bad to take since it reduces the mana cost of essential spells like Power Word: Shield and Prayer of Mending, but the other two disc spells it affects are Renew and Holy Nova, neither of which are worth using very often. I take these talents because they do help you get down the tree, but if there is something else you need to get for a certain fight mechanic, you can sometimes skip this.
- Evangelism I've already explained this talent a little but let me go further. Evangelism is a stacking buff gained whenever you cast Smite. That buff reduces the spell cost of Smite and Penance, and increases the damage done by both those spells as well. The buff stacks up to 5.
- Archangel This talent will take your Evangelism stacks and convert it into a small amount of mana (very marginal) and a very significant healing buff. It's best to save the healing buff for a time when you need to bomb a target with a lot of strong heals (it's nice to pair with a Power Infusion.)
- Inner Sanctum You'll take a point in this if you skip Archangel just to get down the tree, but if you choose to invest more these talents are more for survival. Inner Sanctum allows you to run faster when you use Inner Will, and gain damage reduction to spell damage if you use Inner Fire.
- Soul Warding This allows you to cast Power Word: Shields back to back without having to wait through a cooldown. Though the days of bubble spamming are over due to mana constraints, this talent still allows you to put out as many shields as you have GCDs.
- Renewed Hope Possibly one of our most important talents, Renewed Hope increases our chance to crit with many key spells on targets afflicted with Weakened Soul or buffed with Grace. Because crit heals convert into Divine Aegis shields, this talent contributes greatly to our healing output.
- Power Infusion One of discipline's long time signature abilities, Power Infusion lets you cast at increased speed and reduced mana cost for 15 seconds. You can cast it on yourself or others, but you should usually keep this ability for yourself since it helps to take the bite off expensive spells.
- Atonement This is a must for Archangel disc priests and a pass for disc priests without it. This converts all your damage from Smite into healing, point for point. The healing will hit whoever has the lowest health in melee range of the enemy target you cast Smite on, including yourself if you're soloing. Anytime you gain the benefits of a damage debuff, your healing through Smite will also increase.
- Inner Focus Another must have for disc priests. This makes your next Greater Heal, Flash Heal, Binding Heal, or Prayer of Healing free in mana cost. The talent also increases the chance to crit of that spell. The 45 second cooldown on this ability isn't long at all, and if you're diligent you should be able to continually the ability in combat to save yourself mana whenever you need a fast, large, or group heal.
- Rapture I already talked about how this talent works, so you should know that it is essential to disc mana regen. Whenever one of your Power Word: Shields is consumed or dispelled, you get 6% of your mana pool back.
- Borrowed Time Each time you cast Power Word: Shield you get a 14% haste buff to whatever spell you cast immediately after. This includes any spell with a cast bar as well as instant cast spells. Instant cast spells will have their GCD reduced as opposed to the cast time. The other thing about instant cast spells is they won't consume the Borrowed Time buff, so if you cast your shield then followed it up with an instant cast spell, you can still use a few seconds of your Borrowed Time buff toward another casted heal directly after.
- Strength of Soul These talents are extremely important to disc priests who cast Heal as their bread and butter heal (which technically is all of us, even if you are an Archangel priest. There will be times when you use Heal over Smite.) The talent reduces the duration of debuff Weakened Soul each time you cast Heal on a target afflicted by the debuff. This means you can cast another Power Word: Shield on your target sooner.
- Divine Aegis This talent is a huge part of your healing output. It converts a 30% of each of your critical heals into a shield which can stack up to 20% of your health. These talents are why it's so important to have crit as a stat since it grants your spells the chance to crit. Prayer of Healing also works with this talent, and rather than requiring your spell to crit, each target who is healed with Prayer of Healing will get a Divine Aegis worth 30% of the healing done, crit or not.
- Pain Suppression This ability reduces the incoming damage on a target for a flat 8 seconds, that means it's great for any ability used against a target or tank that has a duration (as opposed to being a single, lethal strike.) Be careful when you use this ability because it does have a long cooldown.If you're in a raid, you should talk with your raid leader about the best time to incorporate the ability into the fight. If you're by yourself just make your best call. A tank won't go from mostly full to zero in the blink of an eye, so you should have time to see that the damage he or she is taking is too much to heal through and apply the buff.
- Train of Thought These talents are particularly strong for Archangel disc priests since Smite will reduce the cooldown of Penance. The part that reduces the cooldown of Inner Focus has great synergy with ... well, Inner Focus, since you can use that talent ability to remove the cost of Greater Heal.
- Grace Grace buffs healing on your current target after you've cast one to three spells on them and stacked up the Grace buff. Grace stacks up to 3. These talents shouldn't be underestimated because with a full stack your healing goes up 12%. Your chance to crit on the target is also increased thanks to Renewed Hope.
- Power Word: Barrier This is disc's big new talent, and it's a lot like getting a second Pain Suppression, except it's for a group. While under the shield it also prevents interruption of spell casting. Snazzy, huh? Just like with Pain Suppression you'll need to be careful when you use this ability. Consult your raid leader in a raid or use your best judgment when you're in a party.
- Empowered Healing This talent buffs most of your healing spells which is great, but depending on how many free points you have, you don't need to max it out. I only put as many points in as I need to get Inspiration.
- Divine Fury Regardless of your spec you need to take this talent. It reduces the cast time on both Heal and Smite significantly.
- Desperate Prayer Though this is a strictly survival talent it can sometimes be what you need if you get intentionally or unintentionally separated from the group and need healing. It's great for emergencies.
- Surge of Light I thought the proc on this ability would be bad but I was surprised to find it's not too bad. You'll have to cast either Heal or Smite to proc it though, so if you didn't take Archangel and Atonement, but are rolling with a Greater Heal heavy palette, you may want to skip this.
- Inspiration If you're healing a tank you want to get this. Everything you put on the tank, whether it's a shield, a buff, or a heal, is one more thing to help keep him and your group alive. Quite important, don't you think?
- Darkness If you value some extra haste and you've got extra points to spend, this is a worthwhile option. Even one point in this talent is better than nothing.
- Veiled Shadows You can take this if you plan on using Shadowfiend early in combat and would like to have a second use available later on. Overall though, I don't recommend this because you're not going to be getting two, fully effective Shadowfiend cooldowns in a fight just because you reduce the cooldown by a minute. Hymn of Hope will still have a longer cooldown that won't be available the next time Shadowfiend is; plus if you use Hymn of Hope when you're not out of mana you won't get the full effect.
Healing as a discipline priest
There following are some general healing tips for discipline priests.
- Always use Prayer of Mending on cooldown. It's one of your best spells. The only time you don't want to use it is when you are completely out of mana and trying to still do some lasting contributions to the fight; in those cases, use Power Word: Shield so you can get mana back at the same time.
- Always use Power Word: Shield on the tank when he is not afflicted with Weakened Soul. The tank is the best target to invest your shields into in order to maximize your mana return from Rapture.
- Use Penance as much as possible unless you expect a certain mechanic in the encounter to occur that you want to counter with Penance. In general, player health will not yo-yo as much as it did in Wrath, so utilizing Penance is a lot about efficiency.
- Use your Power Infusion on yourself. Unlike Wrath, you'll want to use it for more than just its haste abilities. The mana cost reduction is ideal to use right before you know you need to spam a bunch of expensive spells like Flash Heal, Greater Heal, or Divine Hymn. The haste bonus makes it especially good for use with Greater Heal's slow cast time.
- Don't spam anything but Heal if you can help it. Spamming shields is typically a poor choice unless you know a specific mechanic is going to come up where the shields will be consumed.
- It's extremely hard to overheal now because players in party and raids usually are not at full health; this makes Prayer of Healing extremely efficient most of the time. Prayer of Healing also has a guaranteed Divine Aegis proc on every target healed, which means it's even better than it appears. If you're in a 5-man using the Glyph of Prayer of Healing, it gets better. This means disc priests should have no qualms using Prayer of Healing now, but they should still be weary of spamming it all at once since that will hurt your longevity. You only need to heal players who are at risk of dying, not players who aren't at full. So save Prayer of Healing usage for times when you have Inner Focus or Power Infusion, or when you absolutely need to top the group off for incoming group-wide damage. Your Divine Aegis shields will be wasted if your targets don't take more damage after you apply those shields.
- Plan on saving Pain Suppression and Power Word: Barrier for specific mechanics in a fight. In a 5-man, this is generally for when the tank takes increased damage. In raids, work with your raid leader and tanks to decide the best time and place to use these abilities.
- Avoid Flash Heal unless you absolutely need it to save the day. It's one of the least efficient spells you have.
- Also avoid Renew unless a tank is getting wrecked with damage. On a tank, the HoT will help to stabilize incoming damage, but on targets not taking immediate and direct damage, a HoT is a poor choice to get a player's health up.
- Think of Holy Nova as a utility, not as a heal. The healing on it is extremely poor.
- Develop a firm understanding of what talents work with what spells. After you Smite, watch your Penance cooldown because it will be shorter than normal. Also plan to use Power Word: Shield more if you're casting Heal a lot, but you won't cast more than two Heals in a row if you take Strength of Soul.
- Use Inner Focus as much as you can.
- Remember that targets afflicted by Weakened Soul (and Grace) have increased crit chance, so if you're planning some AoE healing, you might want to lay down some extra shields before you cast, particularly if you're expecting lots of incoming damage.