Toay we steamed through Utgarde Keep on an easy badge run, it occurred to us that it's only fairly recently that we could heal through at such a pace without running out of mana or allowing the whole group to become dirt inspection officers time after time.
As you can see, our professions are in a dire state. We thought we'd get that over and done with before we get to the helpful stuff. Long story short, we like healing and fighting, we don't like cutting stuff off dead animals and we can only power level Enchanting for about 20 minutes every 3 days without feeling despondent and irritated.
First things first, Plate is best. Being a Holy Paladin, one of the greatest things is that you're covered in armor and thus don't take much damage as well as you can still heal yourself for your entire health pool in 2.5 seconds. When we started running Heroics we made a pledge to ourselves that we would stick to plate, as it's the best. That attitude disappeared when the Battlemap Hide Helm dropped. It's leather and yet it is still awesome. We made an exception for this item. Then Dalronn's Jerkin dropped, so we made an exception for that too. And as you can see, we started making exceptions all over the place. The moral of the story is: If it's an upgrade, take it! Don't be put off by the casters; don't be brought low by the Warlocks, Mages, Shadow Priests, the Shamans and Druids at that.
We've never claimed to be an expert on the Paladin class; we're just an enthusiastic amateur. Still, we've been PUG'ing with Grim like an addict for about a week and a half now, so we can tell you from personal experience that to power through Heroics without having to let people drop all the time. See the following tips:
The first tip for you is that you at least need two hundred mana every 5second, with Blessing of Wisdom up. If you run out of mana, you run out of answers. The group dies and you feel silly. Make sure you use Divine Plea all the time! Whenever it comes up, use it. It is your most valuable tool. If you have a trinket that grants spell-power, use it when you have Divine Plea going. Also, keep Sacred Shield up on the tank at all times, that extra 50% chance to crit with Flash of Light is invaluable. It provides a good amount of mitigation as well.
The second tip is crit chance. Make sure you have at least 20%. Crit chance for Holy Paladins is a very important stat. We get a 60% mana refund on all of heals when they crit, which can, if you combine it with Divine Illumination, make us one of the longest lasting healers in the game.
The third tip is Intellect. You need a lot of it. We'd suggest trying to have about 800 to 900 Intellects, closer to 900 preferably. It increases your mana pool and your crit chance. A bigger mana pool is always something to be chased. No matter how lucky you are there will always be at least one occasion when you are low on mana. It's unavoidable, but it happens.
There is a bit of debate raging as to whether Haste or Crit is the way to go. We'll be honest with you, it's down to you. If you want to be able to lay down 4 Holy Lights in rapid succession, go for Haste. If you want to make your one Holy Light land for a staggering amount, go for Crit Chance. We wouldn't chase Haste gear, if it comes on a piece of gear that's an upgrade, go for it. If you're dropping other stats for it, it's down to your own judgment. Judgments of the Pure give us a nice 15% increase in Haste rating anyway.