Based on my own fairly extensive look at stat balance for Holy, here is my breakdown.
Intellect - Its always better. Period. Try to gem it, enchant it, and everything else everywhere possible. For socket bonuses, if its intellect, go for it. If you trade 10 intellect for 20 of another stat, its worth it, but you should probably avoid picking up a bit of spirit for a loss of 20 intellect. In the end as long as you always try to get more of this, you're doing fine, you can't really go wrong.
Spirit - You need it to cast more spells. At some point you may find yourself never running low on mana, and if that happens in the highest intensity healing you generally ever encounter (really tough bosses, or against difficult enemies during quests, or wherever) then you probably don't want any more. Spirit can be forged into, but it will be present on almost all of your gear, so you may just be better off adjusting other stats than reforging into and out of this one. If you have a lot of gear without spirit and you find yourself occasionally going OOM before a fight is over, you may want to reforge into some in that case.
Haste - You will want some Haste. It helps you react more quickly to damage intake, sure, but more importantly it boosts one of your only sources of strong AOE healing - Holy Radiance. The first "plateau" you want to hit with Haste is about 6% (774 rating) which nets you 13 ticks from your Holy Radiance when used (assuming you have all other appropriate haste buffs). Its a good level of haste to start with anyway, so I'd say shoot for this and hold here for some time. Its certainly not WRONG to continue using more haste, but the other stats have value too - don't avoid them because they're a lower percentage point.
Crit / Mastery - These two somewhat go hand in hand. Both boost your output without costing you any more mana, both have their ups and downs, and both support the other almost equally well. Basically, as far as I and most others can tell, you should try to keep these about even for maximum effect. Going with one or the other isn't wrong, or bad, but you may gain more from using both than if you chose to be more focused.
And thats about all there is to it.
TLDR: Int > Spirit > Haste to 6%+ (more in higher tiers of gear) > Crit / Mastery
You can take a look at my stats to see what I am talking about, balance wise.