A Holy Paladin is really fun to play, and a great one-target healer, but nowadays you need to be competitive to be considered for an Icecrown Citadel 10/25 players raid in the sense that you must be properly gemmed and have the right spec.
I used to have a Holy Priest back in the day since 2004, and always got complaints about my healing. Then with Burning Crusade, I was tired of the aggro/death situations in Karazhan, and re-rolled a Holy Paladin. All shiny in plate armor, I felt nigh invulnerable -- always being the last one to die in a wipe.
However, I still heard the occassional complaint about my healing when doing some PUG runs through Heroic dungeons. After so many years playing the game, I learned where I was wrong: Gems and Spec.
Nowadays, Holy Paladins skip the spell power gems, and go straight for
Brilliant King's Amber. Replace all your gems with this to add +20 intellect in all your slots. Make sure to place a +10 All Stats gem into your shoulder slot to activate your Insightful Earthsiege Diamond meta gem. Your mana pool will grow, and you will get some extra benefits into other stats.
You can place this Brilliant King's Amber yellow gem into blue, and red sockets. Sure, you will lose the socket bonus, but your gain anyway. You will never get the usual: "You got your gemming all wrong", and you will feel the difference in your next raid run when you have less issues going out of mana.
The way you assign points into your three Talent Trees will guarantee you are mana regen efficient for long fights against Icecrown Citadel bosses, and that you get the best crits possible and the more consecutive crits in a row..
Below is a screenshot of the ideal Holy Spec: 51 (Holy), 5 (Prot), 15 (Ret).
PROTECTION TALENT TREE
In the Protection Tree, a must have for a healer is 5/5 Divinity: Increases all healing done by you and all healing effects by 5%.
You will notice an increase of healing from your Holy Light, Flash of Light, Holy Shock, and probably the effects from the Glyph of Holy Light, which heals nearby players for 10% of the total heal amount of your Holy Light on your target.
RETRIBUTION TALENT TREE
In the Retribution Tree, some people might think it's a waste to spend 15 points in a non-healing tree, but then upon a closer look you will see it does have its payoff, to do so.
Benediction (5/5): Reduces the mana cost of all instant cast spells by 10%.
Improved Blessing of Might (2/2): Increases the attack power bonus of your Blessing of Might by 25%.
Heart of the Crusader (3/3): In addition to the normal effect, your judgement spells will also increase the critical strike chance of all attacks made against that target by an additional 3%.
Conviction (5/5): Increases your chance to get a critical strike with all spells and attacks by 5%.
You get a cornucopia of good-good bonuses not just for your own healing prowess, but to empower your raid members. The healing benefits from the Retribution talent tree comes from Benediction and Conviction. Your goal in that tree really is Conviction. You will notice a large percent of each Holy Light you cast will sky-rocket to over 18,000 healing crits more often than before -- something really needed to keep your tank alive, and of course, more often crits translates into better mana regeneration for the Paladin.
The raid also benefits from Blessing of Might adding a 25% to attack power needed by the tank, fury warriors, rogues, hunters and some shamans. The good thing for a Holy Paladin in this Retribution tree is that your Judgement of Wisdom will cause you to regenerate mana as usual with the benefit that all your raid gets a 3% critical strike bonus through all their attacks (melee/ranged) against the target.
After wiping my spec, and adding this new redistribution of talent points, I went to the Icecrown Citadel 10-players raid dungeon. I had never been able to kill Saurfang because I would run out of mana when he was around 40% health. With this spec, I still had a lot of mana by the time Saurfang, Jr. died. The good part is there was no wipes. No one died through Morrowgar, Lady Deathwhisper, nor in the Gunship Battle event. Good luck with homework getting those gems and testing this respec..