Healers and You: What to Roll

I see this question asked a lot in the advice channel. Granted, I see a lot of similar questions asked but healers are more my thing so, here's my two cents on the matter.

"What healer should I roll/is more effective/is better in T4?"

The answer to the first question is, and always should be "the one you have more fun with". The second is debatable and the third relates closely to the second. What follows is merely my take on how they measure up when compared to each other and should be taken with a grain of salt, as I am more familiar with the Zealot than anything else.
Be warned! Wall of Text ahead. Plus I did not organize this in any way prior to typing it so... well, enjoy.

ABILITIES

Healing:
Zealots have, from early on, more healing abilities than any other. While DoKs spend quite a while with just two direct healing spells, Zealots (and Shamans to some extent) have a wider choice when it comes to just standing back doing the heals. In fact, DoKs get 5 direct healing abilities at most while Zealots get at least seven that I can think of at the top of my head (correct me if I'm wrong). When it comes to pocketing you might feel more comfortable having a Zealot and Shaman at your back, seeing a DoK has only two single target healing abilities (Restore Essence and Soul Infusion). A Zealot is more likely to keep you HoTed (with TC, LA, DM and maybe even the third ritual) between big heals while DoKs will rely more on their quick-casting group heals and their group absorb and HoT.
Shamans sit somewhere in between. Although they lack the instant gratification spell and have, instead, a very short cast-time heal/HoT similar to Restore Essence, they have what I like to call the "big one-target" that Zealots also possess. Their single-target absorb is also as useful/useless as a Zealot's although it can be made a bit more useful with the use of a certain tactic (although as soon as the barrier fades, which is soon, its bye-bye dmg).

Damage:
DoKs are, by far, the most common of DPSing healers. This could be because they have the sturdiest of armors compared to other healers (Medium Robe vs Robe) or it could just be that everyone loves the way a Dark Elf looks dual-wielding. Whatever the reason, DPS Shamans and Zealots in particular seem to be the most shunned. I've tried the DPS route with all of them and, granted, they all have very different feels to them, but the most fun to play, imho, is the Shaman. Simply because i still have the ability to help support my team while I DPS as it increases the effectiveness of my healing. But if you prefer a more "hands-on" approach to damage, DoKs are the Melee ones of the bunch which also makes them one of the first targets in the front lines.

I can't tell you which of the classes is more adept at DPSing or which has the most damage dealing abilities simply because I've never delved into the matter much. But if you do decide to embark on this venture be prepared to grow a very thick skin. Although it is my belief that DPS healers are completely viable, people expect a healing class to heal. That is the main reason they group with them.

Support:
All three classes have a certain buff they will cast at the beginning of a battle. For Shamans its Mork's Buffer. For DoK it can one of three Covenants, depending on the Mastery they chose (this is a good way to know what to expect from a DoK): Tenacity (they are usually going to be healing), Celerity (usually full DPS) and Vitality (Sacrifice mastery). Zealots have the wider choice in this, having several Marks to buff their group mates with. These are: Daemonic Fury, Vortex, Spell Destroyer and Remaking. Healers expect to be marked with SD and DPSing classes with DF. Tanks are a bit trickier and I usually ask what they want. If not, I give them DF because, lets face it, every tank wants to deal just a bit more damage.
In the midst of battle I consider the Shamans to have the better utility. Zealots have their choice of Rituals (Innervation, Lunacy and Superiority), the afore mentioned absorb and a very basic knockback. Depending on their spec they can also have an AoE stagger. Also, they can increase the wounds of an ally for a short while and silence an opponent. Bu that's it. DoKs have that absorb, and, if Sacrifice specced, they can bestow a damage-to-heal aura. They also have their meager knockback (cone effect) and can reduce their target's parry chance if Torture specced. They can rob AP, nullify buffs, steal strength, silence and snare. But there are too many conditions for these to work for my taste. I find them more useful in 1vs1 combat than in a large scale fight.
Shamans, in my opinion have the best support of all the healing classes. They can give their group mates more damage, cast a GT snare while reducing resistances, rob AP, a healing debuff, a knockback that snares, debuff offensive stats, increase toughness, reduce toughness and most of these skills hit more than one target.

MORALE ABILITIES

Common:
As with every archetype, healers have some morale abilities that are common to every class. Divine Favor is the first you'll stumble upon and it will prove useful for a long time. Until it stops being so and you'll never use it again. Or at least, I recommend you don't. I never used Steal Life myself. Again, I don't think it's worth it.
As for rank 2, you'll probably use Rampaging Siphon much as it heals your entire group and deals undefendable damage (at least I think it is) to possibly a wide range of enemies. As for Focused Mind, I personally have never used it, but it might have its good points. If I remember correctly it makes you impervious to silence and such. That or 0 AP cost. I can't remember.
Rank 3 has just Divine Protection. Bear in mind the shield only works for melee attacks so it's less useful against a bomb squad.
Rank 4 has Alter Fate. I can rarely get this one to work. The radius of it just seems too small. Remember, if you're trying to be a healer you can't quite be in the muck of it where everyone's dying.

Disciple of Khaine:
Rank 1: Life's End - I don't know what this one does but I get the feeling its damage related.
Rank 2: Khaine's Withdrawal - this one removes all debuffs from your entire group. Useful in places like BB and against Engineers.
Rank 3: Universal Confusion - useful for a damage specced DoK. That and for staggering, possibly.
Rank 4: 1001 - This is possibly one of the most famous Morale 4s in the game and if you were ever in a raid where you kept reading "1001 up" now you know what they were on about. Very useful to help damage mitigation. This is available in the Dark Rites (healing specc) tree. Chant of Pain - I don't believe this one is used much as it is only a self-heal. But it beats Alter Fate. In the Sacrifice tree. Vision of Torment - unfortunately, not burst damage, but good for preventing tanks from getting out their good morales. In the Torture tree.

Shaman:
Rank 1: Gork Sez Stop - My favorite morale ability ever. Enemies will rarely notice if they have the debuff and they can literally run themselves ragged after you. I've killed many this way.
Rank 2: Breath of Mork - By the time you get this morale you already have group heals. Good for healing outside warbands or if you really don't like your other morale choices.
Rank 3: You Weren't Using Dat - Another skill that makes Shamans (just like AMs) AP powerhouses. Useful for draining a front line as it is charging at you.
Rank 4: Feelz No Pain - Good for the damage reduction. In the Mork (healing) tree. Fists of Gork - decent damage (for a healer) and a knockback. I'm guessing I'd only use this as a last resort if specced on the Gork (DPS) tree. Steal Yer Thunder - A solid buff/debuff in the Da Green tree (damage-to-heal).

Zealot:
Rank1: Tzeentch's Talon - Or, as I like to call it, the "reverse 1001". It does the exact reverse of 1001 to your target, with the exception of the evade stats. Useful for taking down tanks and PvE bosses.
Rank 2: Eye of Sheerian - My second most favorite morale, simply because I think it looks so cool. Good to add an extra HoT like when you know a boss is going to silence you, for instance.
Rank 3: Suppressing the Fragile Unbelievers - Would be awesome if not for the short range. Again, you would have to be in the thick of it and I, personally, prefer hiding behind a tree.
Rank 4: Tzeentch's Shielding - A very decent shield and everybody loves AP. In the Alchemy (direct heals) tree. Tzeentch's Scream - Another one with AP and maybe a not so excellent HoT. In the Witchcraft (HoT) tree. Windblock - the heal debuff and my weapon of choice for killing Steveo. Yeah, I killed Steveo once and this is how I did it. My moment of glory. In the Dark Rites (group heal) tree.

TACTICS

If I went into a detailed description of all tactics I'd lose whatever little readers I had so I'll just make a quick overview.

DoK: I feel that, as tactics go, they're pretty limited. Either you take up offensive tactics which severely hamper your direct healing ability, or you take the more healing and defensive oriented abilities and you might as well be stabbing people with a butter knife.

Shaman: They have the most coveted of all the healing tactics, at least in my opinion. They also have the equivalent for a damage build. They also have a wide range of utility tactics that gives them a third possible role to play. If you played FFXII (although I weep for your loss if you did) you will understand that I call them Synergists. They also have the ever useful GTFO tactic.

Zealot: Zealot's have many tactics based on the Harbinger which I will net get into because they're about to change the whole meaning/functionality of the thing. One of the most useful Zealot tactics is Blessing of Chaos, that can make all healers more efficient. The tactics bit is the one I'm the most excited about because of this overhaul business.

MASTERY TREES

You might want to study them all in more depth as I cba to do it for you. :)
DoK: They have the Pure DPS tree (Torture), the Pure Healing tree (Dark Rites) and the Lifetap tree (Sacrifice).
Shaman: As above, a DPS tree (Gork), a Pure Healing tree (Mork) and a Lifetap tree (Da Green).
Zealot: Unlike the other two, the Zealots trees are defined by damage/healing type. You have the Direct tree (Alchemy), the Over Time tree (Witchcraft) and the Group/AoE tree (Dark Rites). If this distinction is a blessing or a curse is up to the individual.

FEELING

Every healer has a different feel to it and you'll find different kinds of people behind each of them. Shaman's tend to be more fickle and unstable. They can, and are encouraged, to swap from DPS to healing on a regular basis so its the right choice for you if you'd like to do a little of both from a safe distance and without having to mess with different gear and builds.
DoKs like to die trying. They'll keep moving and stay close to the front line. They won't be afraid to stand their ground against a small force and, if specced correctly, they can take them on successfully. Healing-wise they can reach the highest numbers in scenarios, maybe because they have the fastest group heals in the game (if you don't count the possible instant cast of Gather Round due to the Shaman mechanic) but they feel a bit gimped when healing in the back lines.
Zealots are sometimes viewed as the best healers and worst DPSers in the game. I consider both these statements a lie. They have maybe the smallest DPS potential (but I take it that that's about to change) but  they are also underestimated which can make them deadly. As their healing goes, their heals take longer and are more "to the point" because they have no class mechanic restrictions.  A Zealot is usually gifted with more patience and, even though some call them the "mobile healers", a Zealot will be more comfortable hiding in a bush while he does his business.

IN SUM

What is better in T4? As with all other archetypes, a healthy mix of all of them. Although I tend to see a higher number of DoKs around and the ones I see the least are Zealots. So if you want to fill a gap I would suggest a Zealot (in Karak-Norn at least). They can all be extremely useful in any tier, mind you. The biggest factor in this would be your affinity to the class not exactly the way it is currently engineered.

It always comes down to the "play what you have the most fun with" argument. I hope this gave you a small insight of what you can expect from each class before you invest your time in one of them.