The Holy Paladin in WoW is for the player who likes to have the support behind the front lines. You will never do much in the way of damage, but you will be able to make others nearly unkillable. You will be able to remove poisons and diseases and break effects that root your team in place, as well as a variety of other things. All in all you're a valuable addition to any team.
While Cataclysm changed the world of Azeroth it was preceded by the 4.0 patch, which made wholesale changes to the talent system, powers and abilities, and so on. It became a “whole new world” in more ways than one.
The general way that the new talents system works is pretty much the same as the old system, but many talents were scrapped, other changed, new talents were added, and so on. At level 85 you have 41 talent points to spend as compared to the old system with 71 at level 80.
In addition to this every class gained unique abilities for each of its specializations. Retribution Paladins, for example, get Templar's Verdict as one of their abilities and Holy Paladins will never get that ability, even if they take Retribution talents. The same is true in reverse, of course. Ret. Pallies will never get the Holy specials.
At level 10 you pick your specialization and the special abilities that you get for picking the Holy specialization are these:
* Holy Shock: This Damages an enemy or heals an ally and generates a charge of Holy Power.
* Meditation: Allows 50% of your mana regeneration from Spirit to continue while in combat.
* Walk in the Light: Increases the effectiveness of your healing spells by 10%.
* Mastery: Illuminated Healing: Your healing spells also place a shield on your target that absorbs some damage. Mastery is trainable at level 80 and all specs of all classes get some form of Mastery.
Holy Power is a new (for WoW 4) game mechanic for Paladins. It's pretty simple to use, but is also a big change from previous Paladin mechanics. Here's how it works: certain abilities generate Holy Power points and up to three can be stacked at any one time. These points are then used to power some other ability, with three points of Holy Power providing a much greater effect than just one. Unlike a Rogue's “Combo Points” (CPs) the Holy Power points stay with you, not with the target. You can store them up and use them on the current target, a new target, or yourself.
Mana is an issue with the new world. Previously, at least for “well geared” characters, mana usage was not much of a concern. Things are different now. As you level your Holy Paladin you have to take various things into consideration. At low level it will be easy. Your group will mow things down and the fights will be short. Let people know if you get low on mana, but it will not be much of a problem. You will be doing a lot of grouping if you want to level your Paladin as Holy.
Higher level dungeons are the next step up and chances are that you will not be able to heal everyone all the time. This is even more true in PvP, where you will see everything from short flights to long, drawn out battles. If your mana drops dangerously low you can pretty much be guaranteed that some melee or caster type will take advantage of that situation. Plus you'll have various stuns and interrupts and silences to worry about. Your name will be a bottom of the “damage done” lists, but you will be able to make some damage dealer very hard to kill. Paladin / Warrior teams can be deadly and good teamwork will make PvP fun. Poor teamwork will just make it a grind.
In raids you will have the mix of the short trash fights and the long boss losses. Running out of mana in boss fights is not fun. Many of your talents have to do with increasing the value of your heals or the efficiency of your mana usage, but you will probably have to do a little triage (DPS goes down before the tank) and pay close attention to your mana and your assigned targets. If you can not heal then the raid will likely fail and if you're out of mana then you are not healing.
Healing can be a blast, if that's your game, and with the new world it can be very challenging. The previous expansion, Wrath of the Lich King, was too easy on all classes, but all that has changed with Cataclysm. You have your work cut out for you, but that's what makes it fun, right?