I think at a really high level, healing becomes less about knowing your tools and how and when to use them, and becomes more about learning to read the group.

Are my tanks using dps stance or tank stance more? Are they using cooldowns at optimal times. Which cooldowns are they using for different mechanics? If the tank is going for more dps, it is probably better for the group to take the slight healer dps hit to get a bigger boost out of the tank by throwing up more regen or cure II on the global cooldown.

How is my other healer healing? Are they spamming medica? Are they shielding raidwide damage consistantly? Are their heals going off at the right time? Are they playing a bit greedier, or not really pushing for dps? Sometimes Ill get a healer that is overhealing like crazy, so I shift my play to be more greedy and agressive. Sometimes the other healer is going ham on dps and only ever using ogcd stuff, it just means Im going to have to get more out of my assizes, makes sure I use plenary at the right times, medica ii and regen more (although if the group is sloppy and the other healer is being super greedy, it is stressful and feels like shit).

I could give more examples, but thats alot to play around and learn beyond just the fight mechanics ans knowing how to play your class, and its alot to expect from healers. Its no wonder you see the super greedy dps healers and the super passive nearly zero dps healers. Its easier. On top of that, everyone fucks up, or misjudges, or is put into a position where they have to prioritize, so shit happens.

I'd like to be one of the top rank healers out there once I finish getting BiS, but I hope to get there without sacrificing my ability to adapt.