I have a (holy) paladin and would like to better understand the mechanics for Holy Power. And do you think it has improved your gameplay ?
It doesn't matter so much for Holy Paladins, being more of interest to Ret and Prot Paladins but here is a break-down of how it works, with Holy-specific information beside it.
Holy Power is somewhat similar to Rogue combo points in that they build up over time if certain abilities are used, to a maximum of three charges. For Holy Paladins, these points are generated by Holy Shock and, if you have the talent Tower of Radiance, direct healing the target of your Beacon of Light.
You can use these charges to enable or boost some skills. Holy Paladins would be most interested in Word of Glory, an instant heal that heals for some total based off how many Holy Power charges you currently have.
This mechanic is a lot more useful for Ret Paladins, who I think it is mainly aimed at. They can charge it using Crusader Strike (for a guaranteed change) or, if they have the Divine Purpose talent (which they should), have a 40% chance of getting charges from just about any Ret skill, i.e. Judgement, Exorcism, Templar's Verdict, Divine Storm, Inquisition, Holy Wrath and Hammer of Wrath. They can then use it to charge their Divine Storm (AoE damage), Templar's Verdict (single target damage) or Inquisition (holy damage boost), or Word of Glory if they want/need a quick heal.
Prot gains Holy Power from Crusader Strike and Hammer of the Righteous, and mainly use their charges on Shield of the Righteous, though Word of Glory is obviously also an option for them as well for a quick self-heal.
Holy Power is a new paladin resource mechanic in Cataclysm, which works similar to combo points like the rogue's combo point. Holy Power can be gained through certain abilities and further improved by talents to be unleashed with other abilities. It decays over time.
Paladins will generally want to build up Holy Power until it shines through with a bright yellow glowing effect. Once Holy Power has been built up, it can be consumed to augment existing abilities. For example, Word of Glory can be used to cast an instant-cast free heal. This gives paladins several options for how to spend their Holy Power, depending on the scenario.
It becomes available for Paladins at level 9, prior to learning Word of Glory.
Since the hotfix for Holy Paladins a week ago Holy Power is IMHO crucial for healing. With 3 stacks up of HP you can heal with Word of Glory at no costs saving you a lot of mana. Because of the hotfix, Holy Light won't stack Holy Power any more on the Beacon of Light (probably the tank), so you want to use Holy Shock on cooldown to stack up HP (btw: does anyone understand, why Blizz designed an instant cast to be used on cd?). To sum up: 3xHP to use Word of Glory prevent you running out of mana too quickly (depends on your healing gear).