Isn't it the overall raid health?
This is the giant no brainer. You as the healer go after the players that have taken damage. The players who have full health don't really need you to worry about getting any sort of direct heals on them at all. It turns into overhealing and is just simply unnecessary. But things become much trickier when everyone has taken damage. I'll just throw a simple figure out there. Let us presume that Grumpy (it's one of Arthas' massive dogs you don't know about) just took a bite out of the raid and drops everyone down to 25% (instead of 10% because his teeth aren't as sharp yet).
For a new raid healer, it's going to be extremely overwhelming. There are so many questions to ask yourself and you just don't know where to start. I know my first instinct was to start panicking and just go through each player one by one and heal them individually until everyone was past the 10% line. But when you're healing, try to keep these questions in the back of your head when cycling through.
What is the immediate threat?
Most raid bosses have a tendency to change up their targets. Sometimes they peel off the tank and start beating on the off tank for whatever reason. If you're healing the main tank and your target of target shows a different player, then it's a good bet that the tank you were healing is going to be okay for a while and isn't going to need those heals anymore.
Are there any incoming heals?
There are several raid frames out there which have an ability to communicate with other raid frames. That is, if you and I were in the same raid, and I started casting a healing spell on a target, your raid frame would show the approximate incoming heal.
Have any defensive cooldowns been used?
A few weeks ago, I participated in a "routine" kill of Rotface which wasn't as routine as I thought. You see, I ended up solo healing the entire 10 man encounter from 70% to death. It was a pretty harrowing experience, especially for me. But I made a mistake. During the phase where an Ooze explodes, I made the mistake of not hitting the main tank with a Pain Suppression when I was running out. Had I done that, I would've bought myself some extra seconds of tank-free healing. In other words, with a Pain Suppression active on the tank, I know they'll have a much higher chance of survival and I don't have to heal them as intensely. This frees me up to heal the rest of the raid and bring them back up to manageable levels.
By using a defensive cooldown, it allows me to temporarily divert my attention to other players. It buys time. Don't ever skimp out on using such abilities. Don't "save" it for when you need it. The best time to use a defensive cooldown is the time you second guess yourself to see if you should use it or not. If you have to ask, then the answer is a yes.
Is there any environmental damage about to kick in?
These fall under the category of attacks by the boss but are usually aimed at the raid.
Example: You're in phase 2 of Professor Putricide. He's about to throw out a Malleable Goo. You, the healer, are standing directly on top of the Professor and he hasn't spawned any of his orange vials yet. Therefore, you are in a safe position for now. He just turned and faced the entrance and is about to throw Malleable Goo. A quick glance at the raid frames show that all ranged players (except for a hunter) and all healers are at maximum health. They can withstand a single hit of Goo and survive. But that hunter is out there with 50% health and he won't survive a single shot of it.
That is, unless that hunter was standing in the opposite direction away from the door. If that is the case, he can survive a few extra seconds without a heal since the impending Goo isn't going to land on him.
Make sense? Position of raiders and your ability to perceive roughly where players are will help give you an edge when it comes to keeping your raid alive.
Are they special players
In a few cases, you'll have some special players in your raid performing some kind of function that is necessary for a boss to go down. For example, they get targeted by the orange slime on Professor Putricide and have to go for a run. Or maybe that player has a Frost Beacon on their head during the Sindragosa encounter, right? They'll take the initial hit. You can't heal them directly after it since the Frost Tomb is in the way. But you can hit them with a HoT like a Renew to propel their health up while they're still encased in that chunk of ice. These players need special (but often temporary) attention.
What range are they at?
What's their distance? Can you hit them from where you're standing? How difficult is it to reach them? I once remember noticing a DPS death knight tanking a few mobs. It should have been no big deal. The problem was that he was simply too far away. A quick mental calculation in my head showed that by the time I even got in range, he'd be dead. If he's not within my range, then I hope some other healer will be able to heal him up. I'll simply move on to other targets that are nearby. Sometimes there is just nothing you can do. You can't save them all.
Dead or disconnected players
Dead players or offline players are completely off the priority list.
If it's dead healers, then the duty falls on you to quickly figure out who they were healing and if they're as important as the players you were healing. If a raid healer falls, and you're the tank healer, then you might be able to balance the lives of some DPS that the raid healer was looking after instead of all of them.
Exception: If a Reincarnation or a Soulstone of some kind is being used on a dead player, they'll be alive within seconds. It's up to you and the rest of your healers to follow up with heals and buffs. On my priest, I open up with a shield to instantly extend their health bubble before following up with a quick heal.
Hey, healing can be a tough role. But keep these thoughts in the back of your head when you're doing the healing thing with your raid group. It will take a bit of practice and raid healing certainly matches the philosophy of easy to play and difficult to master.
Want some more advice for working with the healers in your guild? Raid Rx has you covered with all there is to know! Need raid or guild healing advice? E-mail me at firstname.lastname@example.org and you could see a future post addressing your question. Looking for less healer-centric raiding advice? Take a look at our raiding column Ready Check.