Note that this has been edited to reflect the changes in Cataclysm / The Shattering, so the stat weighing is not final and a proper raid-balance of stats is not concrete.
Int provides you with spellpower and a larger mana pool, so it is naturally a strong stat.
Haste is also important, as it allows you to cast your spells faster, and to an extent, reduces the GCD (which makes a difference for instant cast spells).
You get a good amount of haste passively through talents though (9% through Judgements of the Pure, 3% through Blazing Light, plus flat cast reductions on certain spells).
Crit is always nice to have, and it synnergizes nicely with some talents (Crit Holy Shocks reduce cast time on HL / DL, for example).
Mastery (at 80 and beyond) gives your heals a bonus shield equal to a % of the heal amount on your target.
Shield effects have always been quite effective, and this more or less gives you a permanent Valan'yr effect to work with.
I can see this being a very popular mastery, however we'll have to see how well it scales further on.
Spirit is now the go-to stat for mana regen as of The Shattering, and will continue to be so throughout Cataclysm.
There may be a FEW items / enchants out there that provide MP5, but for the most part you're looking for spirit for your regen needs.
It still remains to be seen just how much spirit is going to be needed to sustain throughout hardmode fights (especially with the ability to Judge Seal of Insight AND Divine Plea as decent mana regen abilities), but regardless, Spirit is the new regen stat.
As for leveling, just worry about Int, Crit, and to a lesser degree, Haste.
Early on and leveling, a lot of healing will be done with Holy Shock and Word of Glory, which are both instant.
Haste wont play a big role until later when you need to utilize your Flash of Light/Holy Light/Divine Light spells a bit more.
gaming.stackexchange.com/users/414/TheQ
Some people have been very busy since 4.0.1 has gone live, and I ran across some information today that I thought was really helpful.
In priority order: Intellect is your all in one stat as it since 4.0.1 gives Spellpower, Spell Critical Hit and Mana.
You will no longer be able to obtain Spellpower in items (with the exception of your main hand), and you will therefore have to get intellect in PvP as well to obtain the needed Spellpower.
Haste is a fantastic stat for holy paladins as it allows you to great increase the speed of your heals.
It will be needed in many encounters where a slow 2,5 second Divine Light can easily result in a dead tank.
Haste is only beneficial to a certain point for holy paladins though, as the latency sets a softcap on the effectiveness of haste.
If you have ~50ms you should be aiming to get around 750 haste.
0ms softcap is at ~1000 haste.
Spirit is the new Mana per 5 second in WoW, and almost all the plate healing gear you will meet will have spirit on it.
It's a very good stat for holy paladins now, since many of the mana regeneration spell we had in the previous patch have been nerfed such as Divine Plea, Seal of Wisdom (Seal of Insight), and Judgement of Wisdom (Judgement).
Crit is the same as always, and is still one of the least sought after stats.
You will get plenty of crit from the items you obtain, which means it isn't a stat you should be going for directly like in the above cases.
This is mostly because your healing spell already are quite powerful and a critical hit will in most cases lead to overhealing.
Mastery is the newly implemented stat which for Holy Paladins creates a shield on the target you're healing for 8 % of the amount healed.
When you obtain mastery the percentage of absorbed damage increases.
This is not yet a good enough stat to overthrow any of the others.
The shield is very weak and it is definantly not a stat you should be aiming for though reforging at the moment.
gaming.stackexchange.com/users/329/Billy Jo
Yes Intellect is the biggest, then spell power, then crit, then haste.
During hard raids, you want a constant stream of heals going out, so sometimes it's better to have haste than spell power....
Heals per second (HPS) should be around 5k+ at level 80 ICC raids.
MP5 ratings
(Mana per 5 seconds) are going to be superseded soon (if not already).
Healbot is a good addon as it lets you point and click players to heal in a nice little box and shows you who needs heals.
gaming.stackexchange.com/users/3291/angryITguy
Intellect by far is the best stat.
Haste and Spell Crit are good secondary stats to stack.
gaming.stackexchange.com/users/30/Chris
I have a Holy Paladin and I am lvl 85.
The best way to stack your stats are as followsINT>SPIRIT>
HASTE>
CRIT>MASTERY Intellect is more that just mana pool now
its a do
it all caster stat so it is number one.
Spirit helps you keep a lot more mana for the tight spots and helps refill faster after one of these, this is your second most important stat overall.
Haste is the third most important stat because it decreases cooldown and increase cast time.
Makes you way better healer if its your #3 stat.
Lastly Crit for procs like holy shock, and mastery.
Gems to use are the 20+INT 20+ Spirit purple gem, 40
+ Int red gem, and 20+INT 20+ Haste orange gem.
This is what most people I play with agree with.
My pally has been healing great since I started stacking the right stats.
gaming.stackexchange.com/users/13965/Michael R. Bailey
I've recently swapped my paladin (lvl 80, 3.7k gs) from ret to holy, and the advice I've been given by guildies is to stack int, and then haste.
I've started gemming and enchanting for int and seen a significant improvement.
gaming.stackexchange.com/users/212/Antony
Elitist Jerks says Intellect is by far the best stat.
gaming.stackexchange.com/users/164/György Andrasek
What are the primary stats for a holy paladin in World of Warcraft? Do the stats for leveling differ from those used for end-game?
What are the primary stats for a holy paladin in World of Warcraft?
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gaming.stackexchange.com/questions/1385
gaming.stackexchange.com/users/329/Billy Jo
Intellect is the all in one stat, as well as Haste that lets you cast your spells faster. Crit and Mastery are also good ones to have. Intellect  is your all in one stat as since 4.0.1 it gives Spellpower, Spell Critical Hit and Mana. Haste is also important, as it allows you to cast your spells faster, and to an extent, reduces the GCD. Haste  is a fantastic stat for holy paladins as it allows you to great increase the speed of your heals. Crit  is the same as always, and is still one of the least sought after stats, but it is always nice to have, and it synergizes nicely with some talents. Mastery (at 80 and beyond) gives your heals a bonus shield equal to a % of the heal amount on your target. When you obtain mastery the percentage of absorbed damage increases. Spirit helps you keep a lot more mana for the tight spots and helps refill faster after one of these, this is your second most important stat overall. You will no longer be able to obtain Spellpower in items and you will therefore have to get intellect in PvP as well to obtain the needed Spellpower.  
Intellect  is your all in one stat as since 4.0.1 it gives Spellpower, Spell Critical Hit and Mana.  Int provides you with spellpower and a larger mana pool, so it is naturally a strong stat.   Intellect by far is the best stat. Haste is also important, as it allows you to cast your spells faster, and to an extent, reduces the GCD. Haste  is a fantastic stat for holy paladins as it allows you to great increase the speed of your heals. Crit  is the same as always, and is still one of the least sought after stats, but it is always nice to have, and it synergizes nicely with some talents. Mastery (at 80 and beyond) gives your heals a bonus shield equal to a % of the heal amount on your target. When you obtain mastery the percentage of absorbed damage increases. Spirit helps you keep a lot more mana for the tight spots and helps refill faster after one of these, this is your second most important stat overall. You will no longer be able to obtain Spellpower in items and you will therefore have to get intellect in PvP as well to obtain the needed Spellpower.   If you have ~50ms you should be aiming to get around 750 haste. Heals per second (HPS)  should be around 5k+ at level 80 ICC raids.