HPS vs. HPM
There's a sort of zen relationship between the two main concepts that healers deal with: longevity and throughput. Spirit versus intellect. Mana versus spellpower. Healing per mana (HPM) versus healing per second (HPS). The relationship is at the heart of every decision we make, and finding the right balance is the most important aspect of healing today. We obviously can't afford to use such weak heals that our tanks die, and we similarly can't just spam a powerful heal and then watch our mana pool hit zero midway through an encounter. We obviously want to use Holy Shock on cooldown to generate holy power points, but what should we do with the rest of our casts?
Flash of Light owns the HPS crown
When talking about maximizing our HPS, there's no spell that can compete with Flash of Light. It's designed such that it will always be our highest HPS spell, regardless of how much spellpower we obtain. In fact, the scaling of all of our spells has been designed to ensure that no spell overtakes another as we gain spellpower. In our current gear, Flash of Light deals about 15% more healing per second than Divine Light does. It's also capable of delivering its healing faster than DL can, making it the obvious choice for healing targets that need healing immediately. Because of Flash of Light's short cast time, we can cast it on multiple targets faster than we could cast Divine Light on all of those targets, making it key for healing up an entire group in an emergency situation.
The issue is that there simply aren't many emergency situations. With proper timing and enough experience on an encounter, there's really no compelling reason to use Flash of Light. FoL has an insanely high mana cost to compensate for its HPS potency, leaving it as our most inefficient heal. I only really use Flash of Light regularly on one encounter: Chimaeron. The fight has specific mechanics that encourage massive healing to several targets in a fixed amount of time, and FoL and our AoE heals are our best tools to handle it. Other than situations where a tank is about to die (sub-25% life), I don't use FoL at all. It's simply not worth the mana, and with today's tanks' increased HP pools, most people won't die before you can cast a more efficient heal.
Holy Light is incredibly efficient
Holy Light occupies the opposite side of the spectrum, with the lowest HPS but highest HPM of any of our spells. Every other heal will get the job done faster than Holy Light will, but they'll all cost more mana to do it. Because of its innate efficiency, Holy Light finds itself occupying the spot of our "auto attack" heal. It's about 25% more efficient than Divine Light and again continues to be the most efficient spell as our spellpower scales upwards. If there's no urgency, you should be using Holy Light. If you're using any other heal, you're wasting mana.
Plenty of fights have lulls in them, and depending on your healing roster and assignments, you're going to be casting Holy Light a lot. To give you an idea on how often I cast it, on average, I am casting two Holy Lights for every Divine Light on most encounters. Even though it's cast twice as much, it still never surpasses Divine Light on my total healing done. It's a weak heal, but it's so cheap that you can't help but cast it all the time. We use it as a buffer to make sure everyone's topped off for whatever boss ability is coming up next and then use our big heals or AoE heals to handle that situation when it arises.
Divine Light is the balanced heal
Divine Light seems really bad on paper. It's just as slow as Holy Light but less efficient, yet costs more mana than Flash of Light and puts up lower HPS numbers. If it's so awful, why is it consistently #2 on my "healing done" charts (right behind Beacon of Light)? The reason is that it fits right where we need it to. It's the proverbial Baby Bear's porridge -- not too hot, not too cold, but just right.
Holy Light won't keep your tank propped up while he's under heavy damage. It simply doesn't have the throughput. Flash of Light will run you out of mana if you don't stop casting it. It's simply too inefficient. Divine Light has just enough HPS to keep a tank alive, while its HPM is reasonable enough to allow you to use it whenever the situation demands in. With Cataclysm's huge tank health pools, we can safely use this heal without fear of overhealing. A big, slow heal would've never worked in Wrath's game of whack-a-mole, but we're not in Northrend anymore.
Bringing it all together
You're going to be using Holy Shock often to generate holy power points, and Holy Radiance and Light of Dawn allow you to handle AoE damage with ease. Word of Glory can be useful to heal someone quickly and is especially important when you're generating extra holy power via Tower of Radiance. On average, I'm casting about 25 Divine Lights, 50 Holy Lights, 40 Holy Shocks, and 20 Word of Glories per encounter. These numbers vary from fight to fight, but in general, these ratios remain the similar. I occasionally toss in a Flash of Light if a tank dips precipitously low, but that's the exception to the rule. I'm casting Holy Light and Holy Shock nonstop to keep life pools relatively stable, with Divine Light reserved for situations where HL simply isn't keeping up.
The Light and How to Swing It: Holy helps holy paladins become the powerful healers we're destined to be. Learn the ropes in Cataclysm 101 for holy paladins, study the new balance between intellect and spirit and learn how to level your new Sunwalker. Tanking is a job, DPS is a craft -- but healing is truly an art.