Making a thread for 20 classes would probably not go over well with the mods, so let's see if four threads for each Archetype work.
The Three Types of Hats
While the game has six healing classes, there are three main mechanics that drive these classes. Basically, three types of hats for each side. They are not, however, simple mirrors. While a Zealot and Runepriest may share mechanic and many spells, there many small differences that add up to make them fairly distinct from each other.
Yin and Yang
The Archmage (Order) and Shaman (Destruction) share a back-and-forth mechanic that empowers one aspect of their spellbook by repeatedly using a different aspect. Each time you cast heal, you build a point towards damage spells. You can cast a slightly more powerful damage spell right then (which resets your points) or keep cast healing spells to build a max of five points. Each point reduces the cast time of the affected spell by 20% (so yes, five points means an instant cast) or it'll increases the potency of an instant cast spell by 5% per point.
These classes are good if you like damage and healing, but don't want to be forced into either. You aren't severely penalized if you just heal or just hurt people, you simply miss out on the bonus points. A damage dealer can toss out nukes and drop a fat heal when needed, or a healer can focus on keeping allies alive while peppering the field with potent DoTs. The classes are flexible and have become fairly popular because of it.
Zealots and Runepriests can "mark" allies, which not only provide an hour-long buff, but unlock unique spells the marked player can use.
I have no played either class extensively, but most people say that they are more like traditional healers. Whereas the other healers have masteries divided into healing, damage and (de)buffing, these two focus on AoE, Direct and (de)buffng. I've heard dps is lower, but I've also heard that it's potent when supplemented with their various buffs and debuffs.
For the Warrior Priest and Disciple of Khaine, healing requires a second resource: Righteous Fury or Soul Essence depending on class. While this resource regens out of combat, in combat you will either have to use your special melee moves (or a weak ranged move) to regen. The only other way is to convert your Action Points into the resource. Both classes have standard heals and in particular have excellent group healing as they get into later levels. However, they several of their heals involve stealing life from enemies, which sets up a variety of options. You an focus on your more group-oriented healing, or have the option of healing while dishing out damage at the same time. And of course both classes have potent damage capabilities along with several auras they can activate to buff their group.
This is a good option for people who want an excuse to hurt people. In most cases, you can't get away with just healing and HAVE to damage people. You even have the option to keep on damaging while you heal. But, the class has several requirements for healing (can't melee an enemy? No healing!), so if you ALWAYS want to be able to someone out of deep shit, this isn't the class for you. Many times will you sit on the sidelines, unable to heal a guy because going in to melee would be suicide.
Stats for Hats
: A very important stat. This increases our raw healing. Make sure to get plenty. One caveat, and it's big: WILLPOWER DOES NOT WORK ON LIFE-STEALING EFFECTS. THE ONLY REAL WAY TO IMPROVE LIFESTEALING IS TO INCREASE YOUR DAMAGE. IF YOU PLAN ON HEALING A LOT VIA LIFESTEAL, AVOID WILLPOWER.
As a healer, you will be targeted. Toughness reduces damage taken. Pretty self-explanatory.
Less important for the ranged guys, but Disciples and Warriors will need to keep this in the back of their heads, as they will be easier for enemy melee to reach.
Useless for all except Warriors and Disciples. Increases damage AND, as I said before, increases the healing your life-stealing skills will do.
Damage boosting for Zealots, Shaman, Runepriests and Archmagi. You may or may not want this depending on your playstyle/build.
Burst damage can end you quickly. While toughness is good for reducing damage, Intiative reduces the chance you'll be crit. Keeps your incoming damage more stable and predictable, which is good.
For Warriors and Disciples. Armor pen will allow you to take down tanks more easily. Armor also will reduce your life-steals, so this is another way to bolster such healing.
Bright Wizards deal elemental damage.
A hotkey to target yourself is quite useful. Something that is annoying, however, is that Middle-Mouse buttons do not seem to respond if you are holding down your Right Mouse button. I wanted that as my self-target key, but alas, it's not very viable in many cases. Another good one is to make sure your previous Friendly target key is accessible. When you finish healing yourself, you may want to get your focus back on a tank or other guy taking heat.
Potions that restore Action Points are useful. Pretty common, lower cooldown and allow you to still guzzle health pots. Don't ignore them!
Every healer has a detaunt that will reduce the damage they take from a single target by half so long as they don't further attack that target. These are incredibly useful and work in RvR! DoTs will not break the effect...at least if you casted the DoT before the detaunt. If someone is on you, a smart thing to do is to detaunt before you heal. And make sure the person you are detaunting isn't the focus of the attack unless things are dire for you. You want to help take down that guy, so detaunt someone else if it looks like he'll go down quickly.
Every class gets a debuff removal spell. Don't forget it. Also: it seems that root effects have a 1.5 second minimum duration. You will not be able to remove it until 1.5 seconds have passed. So, if you fail to remove that root...try again.
Watch your six: you cannot walk through any player, so enemies can trap you if they sneak in from behind.
More to come...maybe.