Healers

             For this entry I will expose the pros and cons of every heal spec in WotLK, enjoy.

Holy Paladin

Pros

• Best single target heal in the game in Holy Light.
• Strongest single target heal and an absolute tank saver in Lay on Hands.
• Best Tank healing buff (Beacon of Light)
• Absolutely huge mana pool, which makes Replenishment buff and other % mana restore effects restore a lot of mana.
• A reliable and easy to use passive healing effect in Judgement of Light.
• The Auras along with Aura Mastery can help in situations of extreme raid boss AOE.
• A one minute cooldown mana restoration effect.
• Avenging Wrath grants very powerful heals for 20 seconds.
• Survives more than other healers thanks to Divine Shield.
• Cleanse is very good, magic and disease are important effects to remove in 1 GCD.

 Cons

• Very poor AOE heal. Holy Light glyphed helps a ton, but the 8 yards radius restricts it to melee and tanks mostly.
• Holy Light is the main healing spell used, and it is very costly. Makes necessary to stack intellect through gemming.
• Because it requires a lot of intellect stacking, spell power is lacking which makes the scaling of Holy Light somewhat poor.
• Judgement of Light does a lot of the total healing done by a healer paladin, and it can be used by any Paladin regardless of spec; this makes the holy paladin not a must if you have another paladin in raid.
• Although Divine Plea restores a lot of mana, the chances of using it are rather few. The principle is simple: If you cast a lot Holy Light it means the raid is taking more damage than desired which takes away the chances of using your DP, if you decide to use it anyway it will put a lot of stress on the other healers and risk a wipe.
• No heals over time and only Holy Shock for an instant heal (6 seconds cooldown). It could be said Paladin can't heal while moving, which is a huge downside considering many bosses require a lot of positioning.

 tl;dr: Best tank healing and passive healing, bad AOE heal, low mobility.

Priests

Holy

Pros

•  Good AOE heal in Circle of Healing, Prayer of Mending and Prayer of Healing. Divine Hymn can save a raid.
• Best life saver spell in the game in Guardian Spirit.
• With the right spec it can heal and move continuously, making high mobility boss fights rather easy.
• Excellent mana regen thanks to the high spirit stat and Shadowfiend.
• If spec'ed for it can be an optional single target healer with some AOE heal in CoH and PoM.
• Can continue healing for a while in case a death occurs.
• Body and Soul talent is extremely useful. 
• The Hymns are amazing.

Cons

•  The single target heals are rather weak compared to others. Greater Heal cast time is long even including talents, requires lots of serendipity stacks to make it faster cast but you would have to cast Flash Heal 3 times or Binding Heal 2 times beforehand.
• Sadly, the AOE heals of Holy Priest have cooldown. Even though the cooldowns are rather short (6 sec CoH, 7 sec PoM), the fact that you have to use your precious Global Cooldown for filler Flash Heal or Renew, cuts a lot your HPS (Heal per second). You could use Prayer of Healing while your other heals are in cooldown but it has the same problem than Greater Heal: 10 years cast time. Therefore Holy Priest is not a premier AOE healer either.
• Body and Soul talent is useless if a Discipline Priest is in raid.

 tl;dr: Not the best at single target healing, neither at AOE healing. Mixed healer.


Discipline

Pros

• Prevents damage in absorb effects, which is useful when healing reduction is present or for an incoming high damage effect.
• PW: Shield is a very strong protective spell for an instant cast, and can heal if glyphed.
• Critical heals often cause overheal, but Divine Aegis fixes that so they don't go to waste as much. It effectively converts 30% of your critical heals into a shield. Doing some math at 3 points it increases the bonus critical heal by 90%.
• High Mobility.
• Strong single target heal in Penance.
• The 25% haste bonus from talents makes reaching the haste cap an easy task, allowing higher investment in spell power and critical that other healer specs cannot give the luxury to.
• Excellent life saving cooldowns in Pain Suppression and Power Infusion.
• Still good Divine Hymn and Hymn of Hope to prevent a wipe.
• Prayer of Healing is reliable for AOE healing as long as you use it with a 25% bonus haste charge.

Cons

• Weakened Soul Effect will force you to use single target heals if your shield broke too early.
• Single target healing through Penance is good but the 8 seconds cooldown don't make it a very reliable single target heal.
• Flash Heal spam drains mana fast.
• Prayer of Mending has 10 seconds cooldown compared to Holy build's 7 seconds, reducing its potential.

tl;dr: Too good for reducing damage and preventing a death, bad for extended single target healing.

Restoration Shaman

 Pros

• Earth Shield is a reliable passive tank healing buff.
• Riptide instant and passive healing are very nice,  low cooldown as well. Makes your best heal Chain Heal stronger.
• Chain heal is the best AOE heal spell in the game; no cooldown, 40 yards range, each jump can apply Earthliving and gives stacks of Tidal Waves, which makes your Healing Wave an excellent tank healing spell. The mana cost is also reasonable unlike Prayer of Healing and Holy Light.
• Ancestral Awakening helps with your critical single target heals. It can convert your overheal into actual heal, which is amazing, and saves someone low health if no other healer could get to him/her in time.
• Cleanse Spirit removes curse and disease, the most important effects to remove in WotLK raiding.
• It can resurrect on its own if the worst scenario happens.
• Mana Tide totem is an amazing mana restore totem, 24% to each of the shaman's party members over 12 seconds. Assuming the shaman has mana users in the same group, that makes it an effective restoration of 120% mana, much better than Hymn of Hope because it allows the shaman to continue casting heal spells.

Cons

• Chain Heal has a rather long cast time. Many shamans stack 20 haste gems everywhere just to have it cast faster, but that also makes your heal per cast weaker.
• Because of the necessary Chain Heal spam, shaman lacks some mobility, similar to Holy Paladin.

tl;dr: Resto shaman too op.

 Restoration Druid

Pros

• High Mobility thanks to many instant Heal Over Time spells. 
• A 6% bonus healing buff makes everyone feel a little safer.
• Wild Growth is a decent AOE HoT spell, the cooldown is low enough and acquires targets regardless of the group they are.
• Tranquility is extremely strong, the best AOE life saving heal.
• Swiftmend is a very strong instant heal compared to most other instant heal effects.
• Nourish is fast cast and strong enough to focus your heals when needed.
• Nature's swiftness helps when your Swiftmend is in cooldown, just use it with Healing Touch in a Macro.
• Good for bosses with raid wide damage effects.

Cons

• Resto Druid is best at raid healing, when focusing heals on the tank it sort of falters because Healing Touch has long cast time and Nourish drains mana fast.
• Not exactly the best to save a low health from death, if Nature's swiftness is in CD and the target lacks an available HoT to use your Swiftmend, chances are that player will die.

 tl;dr: It's nice as long as a player doesn't take too much damage in a short spawn of time.

Summary

           As you can see every class has its ups and downs, although shaman seems to have the least downs, doesn't mean it is perfect. I didn't explain the how-to for each class because the objective of this entry was simply compare each class strictly on healing terms. Some classes didn't receive credit for the buffs they provide, this should be accounted for too when building your raid. Regardless of the Healer remember all heal classes can also DPS, in case healing didn't work out for you.