In Vanilla, BC, and even WotLK, healers have had three questions to answer: "Who do I heal?" "What spell do I use?" and "When do I cast it?"
Obviously, there are several skills involved in healing - reflexes used to play a role, triage plays a role, et-cetera, but most of these can actually be boiled down to a single skill: Predicting the future.
In order to select the correct heal, a healer has to read 2.5-5.0 seconds into the future to see if the tank will still be alive when your Heal lands, and if s/he'll have time to cast a Greater Heal after that. A healer with five-second-clairvoyance can answer the first two questions 100% correctly, 100% of the time. (Obviously, it took even less in Wrath)
The first two questions also have a finite number of answers. There are only so many people in the group, and for any particular person (or group of people for AoE spells), there are only 1-3 heals anyone would really actually consider healing them with. Even without actively thinking about the future, it's fairly easy to learn by observation what heals to cast in what situations.
The third question is much harder. Cast a heal too soon and it overheals. Cast it too late and the target's already dead. Between the two, there is a range to select just the right moment in order to maximize the time spent outside the five-second-rule.
In order to maximize time outside the 5SR, a healer had to see the entire fight and then plan a strategy around it. Even a healer with 10-minute clairvoyance would also need a strategic mind. It would sometimes be a better idea to start casting a Prayer of Healing two seconds after another heal and not dodge the 5SR at all in the short term, if doing so would allow the healer to take a nine-second nap afterward.
Obviously, the answer to this question affects the other two as well. We might use a Renew instead of a downranked Heal in order to cover for damage that would be taken later.
It is no accident that the healers which used all of their spells rather than just a couple (Spam of Light and Brain Heal) were the ones that cared about the Five Second Rule.
Cataclysm inflated health bars, making the range of when we can cast heals very wide, and removed the five-second-rule, so the exact time within the range completely unimportant. This means that healers now only have to answer the easier two questions: "Who" and "What." "When" no longer matters.
So in Cataclysm, we only have the first two, and they're not hard questions to nail 99% of the time. This creates an extremely low skill cap on healing.
Give me back my five second rule. Do not make me a spam-of-light Burning Crusade paladin.