Throughput can be summed up simply: a measure of a healer's ability to put green numbers on the screen. While we can make the numbers bigger via spell power or more rapid via haste, the concept remains the same. Often simplified to 'Healing per second' or HPS, throughput is vital to keeping your target alive in high damage situations. Your HPS must be greater than the target's incoming DPS, or else you'll fall behind and your target will die. Haste smooths out your HPS so that you have a more even stream of healing, and is particularly valuable when the tank is taking hits rapidly. Spell power bumps up the size of the heal, which is more valuable when your target is likely to get hit hard but not very often: think of a powerful boss AoE ability like Decimate.
While it can be described several different ways, I like to think of longevity as a two component system: the base mana tank and the in-fight regeneration. Your initial mana pool at the start of a fight becomes the baseline for how much healing you're capable of, with regeneration adding on to that as the encounter progresses. If you had a mana pool of say 30k, and your Holy Light cost about 1,000 mana (not actually), your initial longevity would only be 30 Holy Lights. Luckily, through a variety of sources, paladins are able to regenerate massive amounts of mana, which further extend the amount of healing spells we can afford.
Let's examine our primary source of longevity: intellect. Intellect is the best longevity stat in the game, due to the fact that it not only boosts our base mana tank and in-fight regeneration, it also touches nearly every healing stat we have. It boosts our initial mana pool, our spell power, our critical strike rating, and our regeneration via Divine Plea, Replenishment, Illumination, and Seal of Wisdom. Due to its diversity, intellect really trumps every other stat option. Divine Plea can be one of our biggest mana regeneration sources, and it only scales off of intellect. MP5 is great, but it only boosts our in-fight regeneration, and not a single thing else . By stacking our gear towards intellect, our longevity increases exponentially, and we even see a small gain to throughput while we're at it.
What every paladin wants:
A paladin's overall goal is to have the ability to put out high HPS when needed, and to put out that high HPS for as long as possible. While exactly how high and for how long we must generate this HPS stream depends on the encounter, it is easily understood that 'more is better'. So how do we mix throughput and longevity to create a healing system that can handle any situation?
Due to intellect, it is so incredibly easy to gear a paladin for nearly infinite longevity. Once achieved, a paladin's only hurdle becomes doing enough HPS to keep a tank up under heavy fire. Our gear and intellect gems/trinkets will handle the mana situation, it becomes our job to use that mana to keep the tanks alive. In addition, there really haven't been any fights of attrition in WotLK, where longevity wins at all costs. In actuality, it's typically the reverse: shorter fights with periods of intense incoming damage. With these parameters and in these conditions, the correct decision becomes obvious: spamming Holy Light.
Now, anyone who tells you that holy paladins just spam Holy Light is doing you a disservice, as no paladin truly casts 100% Holy Lights in any real encounter in the game. Using a mix of Flash of Light for light damage and Holy Light for heavy damage is still necessary, as our longevity is not truly infinite. I find myself splitting the two about 50/50 for healing done in ICC, varying based on which boss we're attempting. Most encounters have lull periods where the HPS requirement simply doesn't demand anything above a couple of quick Flash of Lights. Let me also add the disclaimer that there are paladins in very high-end guild who use FoL builds successfully, but that for the majority of the paladin population, HL builds will be the top performer.
The situation we're concerned with is when the HPS requirements are high, and holy paladins are the one tasked with getting that HPS to a tank reliably. Fights like Festergut in Icecrown can really showcase this, where the tank takes moderate damage some of the time, massive damage for the rest. The approach we take in these high HPS situations becomes the differentiator between a FoL or HL gear build.
A FoL paladin will have stacked the throughput gear necessary to let their FoL do the talking and handle the incoming tank damage. The key problem is that throughput vs incoming damage is not a fight you can lose for long: if your throughput is too low, the tank dies quickly, regardless of how much mana you have remaining. Most of us have had that feeling right as a wipe is happening, where the tank is literally taking so much damage that there's no way anyone would be able to keep them alive. All the mana in the world won't save a dying tank who's incoming DPS is higher than your FoL spec's HPS can handle. You can regain mana via potions, Mana Tide totems, melee'ing the mobs, and several other ways. You can't boost your HPS without using a cooldown.
That's where Holy Light comes in. With the highest HPS of any single target heal in the game, and a cast time and mana cost that facilitate frequent usage, Holy Light becomes the tool with which you can solve all healing problems. Talents like Light's Grace and Judgments of the Pure ensure that our Holy Lights are fast enough to reach their target on time, and the baked-in healing power that WotLK's Holy Light received from the old Blessing of Light ensures that no tank taking reasonable damage will die on our watch. We have amazing longevity due to intellect's wide swath of affected stats, and we have a spell who's HPS makes all others pale in comparison. Holy Light builds become the obvious choice once you've analyzed the strategy behind the decision. Intellect makes it trivial to gear for longevity, and Holy Light makes it even easier to achieve the HPS numbers we require. Now, while FoL paladins will try to reverse the strategy: why not use FoL for its longevity and then try to stack throughput instead? Intellect is the cause: It's far too easy to get FoL down to the haste cap territory, and spell power simply doesn't boost FoL's HPS enough to get the job done. Throughput has no counter-stat to intellect, and so FoL has no ammunition to surpass HL.
Intellect scales so well for us, and with Cataclysm bringing its own intellect-to-spell power conversion, I predict its continued reign as the top paladin stat. As long as intellect remains so valuable, Holy Light builds will be the king of the healing castle. There's no reason to not use your highest HPS spell when you know that your longevity is already taken care of. If we had a truly competitive throughput option, some other stat that could actually compete with intellect, then an FoL build could make sense. Until that day, there's simply no reason not to be casting Holy Light.