Divine Protection rules
When I was first building my Cataclysm talent trees, I wasn't quite sold on Paragon of Virtue. I like the idea of making my cooldowns more usable, but one of my biggest weaknesses is forgetting to use them. Often, when watching videos of myself healing, I will kick myself for never using Guardian of Ancient Kings or Avenging Wrath. Once I started using Divine Protection, I knew that Paragon of Virtue was for me. It was redesigned in Cataclysm, and with PoV, it lets us cut down our incoming damage every 40 seconds. Preventing damage is always more effective than healing it afterwards, and I find that it's easy to just push DP any time a major boss attack is incoming.
Bosses don't tend to do a lot of physical damage to the raid. While Lord Marrowgar did cleave everyone with his Bone Storm, most AoE and raid-damaging abilities are magical in nature. By using the Glyph of Divine Protection, you can cut that incoming magical damage by an amazing 40%. On a fight like Nefarian that has incredible predictable bursts of damage, the glyph outpaces all others. The survivability boost is welcomed, but the mana savings are worth it by themselves.
Assuming that Nefarian's Electrocute normally deals around 100,000 damage, we can count on glyphed DP to cut that down by 40,000, meaning that the glyph reduces 20,000 damage. It would cost us about 5,000 mana to use a couple of heals to bring that health back. The key is that we can use glyphed Divine Protection multiple times throughout the fight, giving us free mana every time. Even if we only use DP on half the Electrocutes, we still saved 20,000 mana, which is more than double the mana return from Glyph of Divinity. Even without the glyph, you should be using Divine Protection to help buffer the incoming damage from any boss attack. Never say no to saving mana.
Most majors are about mana
When trying to judge the value of a major glyph, I like to focus on how much mana it will save me (or grant me). Glyph of Divinity is good for about 10,000 mana, but only once per fight. If you can count on Lay on Hands being available, you're going to get a ton of extra mana when you need it the most. The Glyph of Beacon of Light yields only 1,400 mana every time we switch our Beacon targets, which isn't really that often. I dropped the Beacon glyph from my build recently, and I don't miss it at all. With its 5-minute duration and 50% healing reduction, there's really no reason to change it rapidly. I might save 1,400 to 2,800 mana per encounter with this glyph, which is why it's now gone.
Glyph of Cleansing saves us around 660 mana every time we Cleanse. The value of the glyph obviously depends on how regularly we're Cleansing, which isn't too often in the current raiding tier. On a fight like Cho'gall, I'll end up dispelling around 10 times –- once for each Corruption: Accelerated. For the Ascendant Council, I dispel about 10 times as well. The result of 20% off of each dispel means we'll save over 6,000 mana with the glyph, placing it well ahead of Beacon of Light in potency. The catch is that there aren't that many fights where we dispel at all. In heroic dungeons, however, debuffs are rampant, and I always swap to my Glyph of Cleansing before a long night of running dungeons.
Glyph of Divine Plea obviously takes the cake for mana regeneration from major glyphs. It works out to grant us 3% of our maximum mana per minute, which means it's worth roughly 15,000 mana during a 5-minute fight. We do have to find opportunities to use Divine Plea in order for the glyph to be effective, but with Divine Plea's new 9-second duration, that's not too difficult. I would say that the Glyph of Divine Plea is about as close to mandatory as a major glyph can be.
Light of Dawn versus raid size
The only major glyph that actually increases our throughput is the Glyph of Light of Dawn, which lets our AoE heal hit six targets at once. The glyph is incredibly valuable for healers in 25-man raids since it's easy to hit six injured people with the cone. For 10-man raiders, hitting six players can be more difficult and definitely not guaranteed.
In a way, the prime glyph for Word of Glory and the major glyph for Light of Dawn are intertwined. We have a relatively fixed amount of holy power generation, and we should be glyphing for the holy power release that we use the most. I actually glyph for both normally, but that's because I never remember to use Divine Favor.
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.