Dual spec is the key
Listen, if you haven't picked up your dual talent specialization yet, go get it right now. I'll wait. Blizzard significantly lowered the barrier to entry -- you need only be level 30 with just a few gold in your pocket to purchase the option from any class trainer. While I am slightly bitter at shelling out 10,000 gold to buy dual talent specs on all of my characters a few months ago, I am glad that everyone will have access to two trees now. The new talent tree design has really killed any chance of making a two-purpose build with any success. We simply use our two talent tree options to fill two distinct roles.
Our two focuses are going to be on dungeon healing and open world questing. A standard holy spec is going to be your best bet for healing in the 5-man groups. You shouldn't be too concerned about mana conservation until you start reaching the later dungeons, as your gear at 80 allowed for so much regeneration that you were able to heal through entire raid encounters. While many sources of mana regeneration were nerfed, dungeon healing simply isn't as intense as trying to heal through a heroic raiding boss. You shouldn't toss around Flash of Light for every heal, but between Holy Shock, Word of Glory, and Holy Light, you can always cast something.
Your goal is to use your high throughput to keep your group alive as they learn the new encounters. Always be aware that your incoming damage can spike up at any time, especially if you've never seen the encounter before. A good healer can help buffer the group's mistakes and allow for even the most difficult bosses to be overcome. Pay attention to boss abilities and learn how you can use your own heals and Hand spells to make a fight easier. Don't forget that you don't need to have everyone topped off at all times. Damage comes in much slower in Cataclysm, and so it's not necessarily a bad thing if everyone isn't at full life.
The dungeons and bosses you'll be facing will get tougher as you reach level 85. With every passing level, your haste, crit, and mastery all get significantly weaker due to how ratings scale. By the time you're level 85, your spells take forever to cast and heal for very little. The key is to use the Holy Shock/Word of Glory/Holy Light combo to keep everything under control, and to only drop back to Divine Light and Flash of Light in times of great need.
Dungeons aren't enough
Dungeon queue timers can vary, but on most servers, they're not fast enough to keep you busy all of the time. I like to quest while I'm waiting in a dungeon queue, as it maximizes my experience per hour. While I've talked about the shockadin build a few times before, I never had the opportunity to level in earnest with the spec. When faced with the decision of maintaining multiple sets of gear and choosing a ret/prot spec or simply playing a shockadin, the choice was easy -- I decided to level as holy.
After going through a few quests on my holy paladin using my old shockadin build, I found a few clear problems. While we don't have to worry about it at level 80, once we hit 81 and pick up Inquisition, holy power generation becomes important. Inquisition only lasts for 4 seconds per holy power point, and so we need a way to get to 3 holy power points quickly. I had originally planned to use Tower of Radiance, but I simply didn't find myself healing that often. I went back to the drawing board to redesign the ultimate leveling spec.
My newest shockadin build is definitely not for healing. I eschew core talents like Daybreak and Tower of Radiance, which I wouldn't consider leaving out of a traditional build. Instead, I grab Blessed Life, which is our best friend when it comes to easily generating holy power points. We're going to get beat up all the time, and Blessed Life ensures that those hits are converted to bonus power. Since we're getting beat up, I also snag Eye for an Eye. Denounce ensures that we'll never have mana issues, and Paragon of Virtue helps us work more cooldowns into the mix.
How to level your shockadin
I'd suggest using Blessing of Kings, as we have no need for the mana regeneration from Blessing of Might. Haste is the most important stat you can acquire, and so feel free to pick up any pieces of great haste gear you find while leveling. We want to keep Exorcism's cast time as close to 1 second as possible. Don't worry about picking up any gear with hit rating on it, as the spirit on our gear provides us with a huge chance to hit. I run around with Concentration Aura active, as this helps me get off more Exorcisms when I'm under attack. Don't forget to use Judgement every once in a while to keep Judgements of the Pure activated. I use Holy Shock whenever it's available and go to Exorcism for the rest of my casts.
Using your cooldowns is something crucial to being successful as a shockadin. You may be used to saving them for a tight moment as a healer, but we're playing a DPS class now. I like to rotate through my cooldowns, using one on each pull to minimize my downtime. Avenging Wrath, Inquisition, and Divine Favor can all serve as cooldowns, as well as any clicky trinkets you acquire. I found a great intellect trinket from a quest in Hyjal that also boosts my haste when activated. Use Inquisition whenever one of your other cooldowns isn't available to ensure a swift victory.
Since we're picking up Crusade in this build, feel free to use your massively buffed Holy Light after killing someone to heal yourself. While our multi-target damage may be lacking, we can burst down most enemies quickly. I haven't had any issues so far, and even when I do pull a few too many mobs, I still have the healing potency to keep myself alive. I try to avoid using Word of Glory to heal myself, as I want to save all of my holy power points for Inquisition. The real mantra of the shockadin is "never stop casting." As long as you continue to spam Exorcism, mobs will fall at your feet. Again, as you level up and your valuable haste and crit ratings decay, you're going to get significantly weaker. In fact, level 84 can be incredible rough for a shockadin, as our damage goes down while our opponents' life and damage go up.
The Light and How to Swing It (Holy Edition) helps holy paladins become the powerful healers we're destined to be. Learn the ropes in Holy 101. We can help you keep a tank alive, heal a raid when necessary and beat the global cooldown. Tanking is a job, DPS is a craft -- but healing is truly an art.