The TL;DR is that Blizzard did not change a lot of our actual mechanics with 5.2. Most of the actual changes to our class and playstyle involve gear scaling from the new raid.
Welcome to this 5.2 Holy Paladin raiding guide! This guide is intended to give you the basic and intermediate Holy Paladin knowledge needed to perform in raids. Unfortunately, this isn't a 100% comprehensive guide to everything under the sun, but it's enough to get you rolling and teach you some new tricks (hopefully!).
Let's start with explaining our core heals and Holy Power system, the interaction of which tends to be the bread and butter of Paladin healing.
Holy Shock: This is as close to a "rotational" ability as we have. Because of its inexpensive cost and high critical strike chance as well as synergy with other abilities and talents, this is the cornerstone of our healing. You want to use this very close to its 6 second cooldown, because it's so effective. Generates a charge of Holy Power.
Holy Light: Light, slower single-target heal that is good for periods of low to moderate damage. This won't keep up with heavy damage, but it's still very useful and easy on the mana pool.
Divine Light: Our "heavy" and slower single-target heal. It has a high mana cost, but it also heals for a lot. Generates a charge of Holy Power if used directly on your Beacon of Light target.
Flash of Light: Our "emergency" single-target heal. Very quick and high HPS, but also lower HPM (healing per mana) than Divine Light. Use somewhat sparingly but don't neglect it if someone is on death's door. Generates a charge of Holy Power if used directly on your Beacon of Light target.
Holy Radiance: Our main AoE heal. Heals the target for a moderate amount and heals players within 10 yards of the target. Expensive but worth using (in moderation) in AoE situations. Generates a charge of Holy Power and, with the Daybreak passive, causes your next Holy Shock to have a decent AoE splash on nearby targets. Unfortunately, the short range means that its usefulness can be somewhat limited by fight mechanics and positioning.
Crusader Strike: Generates a charge of Holy Power, but also requires melee range, does no healing of its own, and has a fairly high mana cost for Holy. It also is subject to Expertise, which we will not have much of, so it has a chance to be dodged (or parried, if attacking from the front). Useful at times, but use with caution.
Holy Power heals:
Word of Glory: An instant, fairly strong single-target heal that scales linearly with Holy Power charges (2 HP heals for twice the amount 1 HP does, and 3 HP heals for three times the amount 1 HP does). There isn't a whole lot to say until we get to the talent section, where we learn about the wonders of Eternal Flame.
Light of Dawn: An instant, 6-target AoE heal that also scales linearly with Holy Power. It's one of our few tools to deal with spread AoE damage, and is great especially for burst AoE, but in many cases you will be using Eternal Flame (the modified version of Word of Glory with a 30 second heal-over-time attached, or HoT) quite a bit more, particularly in 10 mans.
One of the interesting mechanics of Holy Power is that WoG and LoD only consume a maximum of three Holy Power, but we can bank up to 5, making us much more versatile when planning around burst damage. For a predictable raid-wide AoE, for instance, it is very useful to bank a lot of Holy Power in order to use a powerful Light of Dawn immediately afterwards, another Holy Power generator, and then another powerful 3-HP Light of Dawn.
Beacon of Light:
This is another core mechanic/buff. When you cast it on a raid/party member, a % of your heals cast on other raid/party members is duplicated and applied to the Beacon target. This is a fantastic mechanic and a staple of why Holy Paladins are generally considered the best single-target/tank healers.
You typically want to keep this on the active tank. Keep in mind that different heals scale differently - Holy Light transfers 100% of the amount healed, whereas Divine Light and Flash of Light transfer 50%. This can be useful strategically, for instance if both your tank and another player are taking a light to moderate amount of damage, it can sometimes be better to use Holy Light on the other player than using a heavier heal.
Most of your AoE healing (including Holy Radiance, Light of Dawn, and Light's Hammer) transfer 15% of their healing to your Beacon target, making your stream of Beacon healing fairly steady regardless of what type of spells you're casting.
Another core mechanic involving Beacon of Light involves a passive called Tower of Radiance. When you cast Flash of Light or Divine Light (your two heavy, expensive direct heals) directly on your Beacon target, you also gain a charge of Holy Power, making these spells much more mana-efficient when healing your Beacon target. I typically don't use Holy Light directly on my Beacon target if they're taking heavy enough damage that DIvine Light wouldn't overheal.
Beacon of Light healing does not apply the Illuminated Healing absorb shields (our Mastery).
Advanced technique: With the Glyph of Beacon of Light, the spell is no longer on the global cooldown, allowing you to recast it every 3 seconds without impacting your other casts. If your current tank does not need healing, it can make sense to swap Beacon targets to another player who does need the splash heal or simply to the person you're about to Divine Light for an extra charge of Holy Power. I wouldn't recommend working on this until you're comfortable with most of the other class mechanics, but it is definitely a very useful mechanic.
You always want to use Seal of Insight. Spell haste, healing, and a small mana return when you get to melee a mob/boss. What's not to love?
For buffing the raid, it's been pretty simplified with MoP, but Blessing of Kings grants 5% Strength, Intellect, and Agility, while Blessing of Might buffs 3000 Mastery.
Blessing of Kings is covered by all specs of Monk and Druid. The Mastery of Blessing of Might is covered by all specs of Shaman, as well as a Hunter pet option, but it's worth noting that Might is far superior to the other Mastery options on any fight where range can be an issue because the other two buffs are either shorter-duration or an aura tied to the player/pet.
Avenging Wrath: WINGS! Okay, this is a staple for any Paladin of any spec, and has been for a long time. 20% damage and healing for 20 seconds, available every 3 minutes. It's a very powerful throughput cooldown, and something you should be in the habit of using whenever you need to boost your healing output.
Divine Favor: Similar to Avenging Wrath, Divine Favor lasts 20 seconds and has a 3 minute cooldown. Instead of a flat healing buff however, it increases your Haste and Crit chance by 20% each. This makes it another amazing cooldown. Given the choice, I would typically use Avenging Wrath for big single-target heals, and Divine Favor for AoE healing (where the crit is a bit more reliable than it would be single-target), but they're both quite strong in either scenario.
Guardian of Ancient Kings: This is our "raid save" so to speak (although it only serves that function to people within a small range). When you cast a single-target heal, the target is healed for an additional 100% and all targets within 10 yards are healed for 10% of the amount healed. It's extremely powerful, but a 5-minute cooldown and a cap of 5 heals ensures that you won't be able to use it more than once or twice per fight. An interesting side-note is that Guardian also increases your haste by 10% per stack, meaning that you can weave AoE heals into your Guardian duration to take advantage of the haste buff without consuming stacks of the spell.
If possible, you want to use 5 Divine Lights to consume your Guardian charges, because that is our biggest single-target heal and thus will splash the most healing. If you have 3 Holy Power, using Word of Glory/Eternal Flame is a good option as well.
Divine Plea: This is our mana regen cooldown. It returns 12% of our total mana over 9 seconds and is available once every 2 minutes. The downside is that while active, your healing is reduced by 50% (that's rough), meaning that you want to use it during lulls in damage where you can slack on healing or even stop entirely. Do use this often though, it's a core regen mechanic.
Hand of Sacrifice: Hand of Sacrifice is our tank cooldown and thus deserves a special mention. It transfers 30% of the damage the target takes to you, which makes it a very potent cooldown when the target is taking heavy damage and you are not. It can kill you in some cases, but with the inflated health pools nowadays it's a lot less dangerous than it used to be. It's also worth noting that damage taken from Hand of Sacrifice is considered magic damage, and is mitigatable by Divine Protection and raid cooldowns such as Power Word: Barrier and Aura Mastery, and you are fully immune to it while Divine Shield is active.
Lay on Hands: This ability has a very hefty 10 minute cooldown, but heals for an absurd amount, making it the best "OMG X PLAYER IS ABOUT TO DIE" button in the game. It also returns a nice amount of mana when glyphed.
Divine Protection: This is our personal defensive cooldown. Baseline, it prevents 40% magical damage taken for 10 seconds (available every minute). Because it's easy to use and off the global cooldown, you should be in the habit of using this often for AoE damage or when you use Hand of Sacrifice. There is also a very useful glyph that changes it to 20% magical AND physical damage reduction, great for certain raid encounters.
Devotion Aura: Raid-wide 20% magic damage reduction for 6 seconds. Very powerful for magic damage (obviously), useless for physical damage. All Paladins have access to this and they don't stack, so speak to your fellow sparklies!
Divine Shield: I think everyone's familiar with this one, but it's our almighty BUBBLE! Use when you need to skirt encounter mechanics, are about to die, or need to cast Hand of Sacrifice to save a tank without risking your own life (this kind of ties into the first two).
Hand of Protection: This is an interesting cooldown, in that it's similar to our own Divine Shield - it not only makes the target immune to physical damage (and unable to deal physical damage) but also clears many harmful physical effects/debuffs on the target. Very, very useful for removing tank debuffs on fights such as Horridon, Durumu, or Iron Qon (all in Throne of Thunder). Since you can only have one Hand spell on a target at a time, be prepared to immedaitely follow it up with the less-painful Hand of Freedom when used on a tank or melee/hunter.
There are other cooldowns you can choose to talent into, but I will cover those in the talent section.
A note about throughput cooldown usage: You will typically want to stagger your throughput cooldowns (Divine Favor, Avenging Wrath, Guardian of Ancient Kings) rather than popping them all at once. This way you cover a larger period of time rather than being REALLY ZOMGPOWERFUL for a short period of time and then having nothing up your sleeve for 160 seconds.