To get in the right healing mindset you need to analyze incoming damage and damage taken by your raid members. It isn’t as simple as just checking this, though. Look at the following factors:
Gear Level: check their gear levels (for max HP, resistances, armor etc.). Classes with higher survival (generally warlocks, warriors, hunters etc.) are second priority when everyone in the raid is taking AoE damage for they have a higher survival to soak the damage with. Your priority should be on those that can get killed above those that generate the most healing done. This varies on your healing style: renew priests generally take targets with lower hp deficits as it is their job to heal targets up to full hp, while other healers with direct healing spells should prevent players from dying (for an explanation see “Assignments & Trust” below).
Classes: warlocks life tap, prot warriors tank, fury warriors taunt for /sit enrage proc and higher threat generation. Such classes generally require more healing throughout the raid. It is easy to see which of them take more damage than others throughout the raid (and at what parts of the raid they take this higher damage). Note that using a meter add-on (SWStats, DPSmate) can help you here by analyzing damage taken and healing taken, as well as death counter.
Positioning: standing in AoE, taking cleave damage etc.. People standing in front of corehounds take more damage; melees take more damage for many npcs do their AoE damage within 30 yards and thus casters are outside their AoE range. Try to predict damage taken by positioning of players.
NPC/BOSS: it is also important to check what you are fighting (for example, lava annihilators in MC will only do single target damage, while firewalkers will burst your raid members down, and dogs/fireguards will AoE the raid. Prioritize your healing accordingly.
Your own positioning should always be top-notch. The reason this is important is because you can minimize walking during raid encounters (and thereby do more healing/be more reliable as a healer). For this you simply need to have an eye on your tanks for ideally there would be a warrior in between the npc you are fighting (even during CC encounters like Garr) and you: they can taunt the npc away from you. This is a big deal because I have often assigned healers to certain targets and they failed to keep them alive because they suck at positioning and have to walk instead of healing.
If you want to min-max your healing squad (for healers cannot be seen as just the sum of its parts, they must be taken as a whole), then you need to be able to rely on them. This means following your assignments. Although it is often advised to help certain people with their assignments when their targets are consistently low hp and they haven’t indicated they can handle it, you should generally leave this up to your raid leader/healing leader to assign more healers to this target (in a proper guild that is, if your leadership is clueless about healing then you will have to improvise yourself). Stealing healing from someone else’s assignment can cause a domino-effect as they start healing other targets for their target is consistently full hp. What’s the problem? Without a dedicated tank healer every pleb healer in your guild will switch over, consistently, to the target that requires the most healing. If the tank is bursted down within this time), then the tank will die. If your guild is bad, then yeah, you will be forced to heal other assignments (for they either don’t adapt their healing assignments when healers indicate they are insufficient or they don’t have healing assignments at all).
If you do this correctly, though, your whole healing squad will do less overhealing, safe more mana, have less downtime and the raid clears the content faster and more smoothly.
Human:
1. The Human Spirit: 5% increased spirit.
2. Sword & Mace Specialization: +5 skill to swords and maces.
3. Diplomacy: +10% more reputation gain.
4. Perception (20 sec duration, stealth detect increased dramatically. 3 min CD).
Priest special skills:
1. Desperate Prayer (instant heal oneself for 1324 to 1563, no mana cost. 42.86% coef. 10 min CD).
2. Feedback (15 sec duration , -105 mana burn/dmg per enemy spell cast on you, 580 mana cost. 3 min CD).
Dwarf:
1. Stoneform (10% armor increase + immunity of poison/disease, 8 sec duration. 3 min CD).
2. Gun Specialization: +5 skill to guns.
3. Frost Resistance: +10 resistance to frost.
4. Find Treasure: searching ability to find treasures.
Priest special skills:
1. Desperate Prayer (instant heal oneself for 1324 to 1563, no mana cost. 42.86% coef. 10 min CD).
2. Fear Ward (10 min duration, 1x fear immune. 100 mana cost. 30 sec CD).
Night Elf:
1. Shadowmeld (instant hide ability, while not in combat).
2. Quickness: +1% dodge.
3. Wisp Spirit: +50% move speed while dead.
4. Nature Resistance: +10 resistant to nature.
Priest special skills:
1. Starshards (936 arcane dmg over 6 sec duration. Gradual dmg increase (15% 2x, 20% 2x, 30% 1x). 350 mana cost. 171.43% coef. No CD).
2. Elune’s Grace (15 sec duration, reduces ranged dmg by 95 and +10% dodge chance. 240 mana cost. 5 min CD).
Troll:
1. Berserking (10 sec duration, 10-30% increased attack speed depending on HP. 3 min CD).
2. Regeneration: +10% increased health regen and 10% health regen in combat.
3. Beast Slaying: +5% damage increased to beasts.
4. Bow & Throwing Specialization: +5 skill to bow and throwing weapons.
Priest special skills
1. Shadowguard (10 min duration, 3 charges of 116 dmg, 250 mana cost. 100% coef. No CD).
2. Hex of Weakness (2 min duration, -20 physical dmg and -20% healing. 240 mana cost. No CD).
Undead:
1. Will of the Forsaken: immunity to fear, charm and sleep for 5 seconds).
2. Cannibalize: 8 sec duration, 4x 7% health restore every 2 seconds while eating corpse).
3. Underwater Breathing: 300% longer breath.
4. Shadow Resistance: + 10 resistant to shadow.
Priest special skills:
1. Devouring plague (24 sec duration, 904 shadow dmg + healing. 985 mana cost. 80% coef. 3 min CD).
2. Touch of Weakness (2 min duration, -20 physical dmg curse on getting hit + 64 shadow dmg. 195 mana cost. 100% coef. No CD).
Looking at these stats, it is obvious which races to choose in PVE:
Alliance: this is pretty self-explanatory (PVP/PVE choices don’t matter):
Dwarf FIRST – because you have 1) each 30 seconds fear immune, 2) high self-heal, 3) 10 frost resist (good for Naxx) and 4) Stoneform. The other choices would be ridiculous since:
Human SECOND – got 5% spirit and mana burn ability instead of Fear Ward + Stoneform (human is arguably better when you assume you are never getting engaged with world buffs while going to BRM/AQ/NAXX for during fear fights you can usually outrage it and you only require 2 dwarf priests for fear ward uptime on tanks during fear fights).
Night Elf THIRD – self-explanatory.
Horde: Horde is a little bit trickier as it is situational on what you want to choose:
Troll PVE FIRST: you got Berserking and Shadowguard (this helps in boss dps when you got aoe dmg going on, but it hurts your warlocks if it removes their shadow bolt procs before being reapplied). Troll is for PVE only first if you are using Berserking actively and properly.
Undead PVP FIRST: the undead just have PVP abilities. However, considering you want to get with world buffs to the raid having a WotF is very nice to have (it also helps in PVE if you suck at positioning during fear bosses).