Superman's weakness is green kryptonite. When confronted with it, he has no option but to surrender. A Green Lantern's weakness is anything yellow, which I suppose means that they hate bananas. Aquaman's weakness is that he can't really do much on dry land, except possibly flop around some. Holy paladins, while not exactly superheroes, have been plagued by a longstanding weakness as well: We've never been able to heal a group with any sort of efficiency or potency.
In Cataclysm, the developers set out to break down the barriers that were preventing holy paladins from being effective group healers. The first order of business was expanding our toolbox to include a few AoE spells, and so we learned Light of Dawn and Holy Radiance. While they've both seen some serious rebalancing and complete redesigns (as well as a rename or two), the final versions of these two spells are now our core AoE healing options. Both abilities tackle different situations, and knowing when to use which is key in being successful as a holy paladin.
Holy Radiance's strengths and weaknesses
Imagine if you could turn your paladin into a Healing Stream Totem -- except that it was only temporary, you could still move around, and it actually healed for a respectable amount. Well, maybe Blizzard's totem example wasn't the best metaphor, but it's close enough. Holy Radiance allows for a paladin (not just holy paladins) to heal all nearby targets over a period of 10 seconds. There are some limitations on its potency, although its efficiency is what makes it crucial in our AoE toolbox.
The key weakness of Holy Radiance is that its healing effect diminishes with distance as well as with the number of players affected. The further we are from our friendly players, or the more players in range, the less healing they receive. The first six players get full healing when we're within 8 yards, but as we move away or add more people, the heal dwindles down to almost nothing. The reduction in potency scales with distance, so each step you take decreases the healing done.
In order for Holy Radiance to be truly effective, we need people to be tightly grouped for the duration. Holy Radiance is incredibly powerful on a fight that has coordinated stacking phases, like Chimaeron or Maloriak. On fights where we need to stay in a spread formation, like the Ascendant Council, it can be less than amazing. One trick to boosting Holy Radiance's effectiveness is to use Divine Favor beforehand. The extra haste from DF means you'll see more HR ticks.
Spread the AoE love
Every paladin has access to Holy Radiance, although through the holy paladin version is far superior. First, we have spellpower scaling to increase the effectiveness, and we have Speed of Light, which cuts the cooldown in half. We also tend to stack haste, which can increase the number of healing ticks we see per cast of HR. Retribution and protection paladins are certainly welcome to leverage their HR during intensive healing phases like Chimaeron's Feud, but holy paladins will be doing far more healing with the ability.
Holy Radiance's throughput isn't enough to heal your entire raid back to full life. A fully realized Holy Radiance, with everyone within 8 yards and Avenging Wrath active, only healed my 10-man group for around 200,000 healing. That's only around 20,000 per player, which is not much in the grand scheme of things. The key is that Holy Radiance only costs slightly more than one Divine Light, meaning that it's an incredibly efficient way to do that 200,000 healing.
In addition, because HR just puts a buff on us, we can continue to leverage our other heals to get the job done. During a particular Feud, I did nearly equally healing between HR and Divine Light. Divine Light's healing cost me nearly five times as much mana, and Holy Radiance's healing only required me to spend a single global cooldown. Holy Radiance isn't going to do the job for you, but it's so incredibly easy and efficient that there's no reason not to use it. Use HR to get as much "cheap" healing as you can, and fall back to the direct heals to finish the job.
Light of Dawn has been neutered
During the early days of Cataclysm raiding, many paladin healers were abusing Tower of Radiance by chain-casting Holy Light onto a tank, and then unleashing the generated holy power points as a Light of Dawn. The so-called "holy flashlight" is a smart heal, so it chooses players with the lowest life first. While spamming Light of Dawn, it's always healing weakened targets, and so it was possible to have LoD do a significant amount of healing. To correct this, Tower of Radiance and Light of Dawn were both nerfed heavily.
Light of Dawn is now one of our weaker heals, although it does play a role in our healing paradigm. Because a portion of each Light of Dawn heal is replayed back to our tank via Beacon of Light (even after last week's scare), healing with LoD actually heals the tank quite a bit. While the positioning requirements behind LoD can be difficult to manage on certain fights, hitting the maximum number of targets is very important. For every target that we miss with LoD, we do less Beacon healing and less healing per holy power point spent. If we can get a good cone that covers most of the raid, then LoD is not too shabby. If we're just hitting one to two people with it, then we should've used a Word of Glory instead.
Shaman have Chain Heal, druids have Wild Growth, but holy paladins are stuck with Light of Dawn to do our active AoE healing. Because it requires holy power points, we're really restricted in how often we can cast it. LoD's weakened throughput and time between casts makes it an afterthought in most healing situations. While you still want to drop your 3 holy power points on an LoD if you can hit the entire group and there are several hurt targets, LoD simply isn't the powerhouse heal that it was in the past. We have to use our direct heals to make up for that when the entire raid is taking damage.
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.