I'm going to start with this one, because it's one of the simplest ways to help out your group. If you're alive, and someone is dead, you should be the first person to resurrect them. Start casting Redemption the moment that you're out of combat, and don't stop until every other player is brought back to life. There are far too many healers who will neglect to resurrect after a fight, and that is the type of paladin that you don't want to be.
We've got tons of haste on our gear, and with Divine Plea, we can always be sure we have enough mana to handle bringing back multiple players. If you're quick to resurrect, that means that your group will have more time to work on the instance, and that you're saving everyone time. As a raid leader, I always look at the 'Total Resurrects' meter to see who's working the hardest to keep the run moving smoothly. The people you are resurrecting will be thankful that you're prompt as well. Also, if your raid leader has called a wipe, you can use Divine Intervention on another healer to save the day. You'll save the guild bank from paying for two repair bills, and you're allowing your group to recover that much faster.
Due to Judgements of the Pure, holy paladins are judging on every single encounter. It's a great way to keep up the 20% haste buff, in addition to being mana-positive if we get a Seal of Wisdom proc. It only takes a single GCD, and we can judge from long distances. We don't necessarily need to judge every time the ability is up, but it can't hurt if we can spare the GCD cost. By using Judgement of Light, we add a significant amount of healing to any DPS class attacking out target. By using Judgement of Wisdom, we ensure that any mana-dependent classes are able to perform at their maximum efficiency. Typically you will want your retribution or protection paladins to use JoW, as maintaining a good JoW uptime is crucial for certain classes' DPS rotations. Of course, this can vary based on your particular group make-up.
If you're the only paladin providing a Judgement effect, try to keep its uptime as high as possible. With a 20 second duration (30 seconds with our T9 set bonus) and only a 10 second cooldown (less if retribution sub-spec), you can keep two different Judgement effects on two different targets at the same time. This is great for keeping something like JoL up on both of the Twin Val'kyr, or making sure that JoW is on both Professor Putricide and his slimes so that your hunters don't run out of mana. There are a variety of mods that can help you monitor your Judgement debuff duration, and maintaining a good uptime will benefit everyone involved.
Lay on Hands a healer
A properly played holy paladin will very rarely have mana issues, as we can almost always just increase our Divine Plea usage. Other healers, however, can run into mana problems on intensive healing fights. If a friendly healer is running low on mana and there isn't an Innervate or Mana Tide Totem available, offer to use Lay on Hands. It will grant your target a significant amount of mana, and allow them to start healing again. I wouldn't use this tactic on a fight where LoH could be important, like Festergut or Putricide phase 3. However, on a fight like Blood Queen Lana'thel, using your LoH on an AoE healer could mean the difference between a wipe and a victory.
Stuns / Interrupts
While paladins don't have any true interrupts, our Hammer of Justice does interrupt mobs if they're casting a spell. If you're in a dungeon and you see a mob casting a nasty spell, use your HoJ to interrupt them and save yourself from having to heal through that damage. You can use HoJ on nearly every enemy in a dungeon, and several trash mobs in a raid. By stunning the enemy, you cause your team to take less damage. You can also use Holy Wrath to stun undead mobs, and this can be clutch if there are several mobs that are casting.
I like to use Holy Wrath on the opening trash of ICC, and you can stop two of the mages from casting their powerful frost attacks, and even interrupt the Nerubian healing spells. While fighting the Val'kyr adds in the center of Icecrown Citadel, you can use Turn Evil on the clones of your healers to prevent them from healing the adds. Don't let the retribution paladins have all the fun: you can use your offensive abilities if the healing is under control.
There are two keys to buffing well as a holy paladin: download PallyPower, and buff often. Using PallyPower, you can coordinate buffing between all the paladins in the raid, and simplify the casting of the Blessings on the raid. If someone dies, always rebuff them with a Greater Blessing. The mana cost is low, and by spending 20 gold on extra Symbols of Kings, you will literally have the thanks of the entire raid. In my opinion, the fewer people that are complaining about buffs, the better. A few gold spent on reagents is a small price to pay for stopping the endless "might plz" spam that will ensue otherwise. Be sure to buff people who are Soulstoned or Rebirthed, as they'll need their new Blessings after coming back to life. You can also buff certain vehicles as well, like the abomination on the Professor Putricide encounter and the drakes in the Oculus. Throwing out a quick Blessing of Kings on these vehicles will give you group an extra boost they would've otherwise never known about.
Speaking of buffs, don't forget about debuffs as well! If there's an effect that you can Cleanse, you should be Cleansing it first. We should always have enough haste for a 1 second spell GCD, and we should be dispelling our friendly targets as often as possible. Don't assume someone else will handle it: dispel it yourself. This is another important metric that many raid leaders use to determine the effort that their healers are putting into a raid, whoever does the most dispelling is seen as putting in the most work. Obviously there are spells that you don't want to dispel immediately, but as a rule of thumb, Cleanse anything you can as often as you can.
Here's the part that I can't assign a value to, and the part that is the hardest to earn. It's your ability to handle situations that are beyond your own capabilities, and to rise above that challenge. It's when your tanks have a miscommunication and pull two packs of trash instead of one. It's when one of your trigger-happy fury warriors steals aggro, and you hit them with a Hand of Protection before they get squished, and then hitting them with a Hand of Freedom so they can start attacking again. It's when you're doling out heals as fast as the GCD allows, and you still make time to toss out a Hand of Salvation to the warlock who doesn't even realize he's at 109% of the tank's threat. It's the situational awareness that allows you to call out for a transition as the patrol is heading right towards you. It's using Hand of Reckoning on the Blood Beast that's about to stomp on a mage's face. It's popping Divine Shield and finishing off a boss' last 1% of life as you furiously mash Holy Light to keep the tank alive.
Being high up on the healing meters is the sign of a good healer. It shows that you understand your role (heal a lot), and that you're not wasting any time. What a meter can't show you are all the things that make a healer great. By paying attention to detail, by taking time out from your duties to make sure the raid is alive and buffed, and by being generous with your abilities, you will set yourself apart as a truly great holy paladin. That's what will get you reinvited to groups and a pat on the back at the end of a raid night. Any paladin can spam Holy Light with a tank targeted, but it takes a true artist to juggle an entire raid encounter's worth of buffs and debuffs and still look good doing it.