This post is to provide an in-depth discussion of gearing decisions for priests. Specifically, I want to address issues of statistical priorities, and therefore gemming/looting goals.
HOLY PRIESTS
My take is that spirit is still king for Holy. Even with the mana return adjustment, Spirit still provides good amounts of both power and endurance. Spellpower is also important, though not to the point that other stats should be lacking; but only in gemming do you have to give up a stat in order to get more spellpower. With rare exceptions, spellpower scales linearly with item level.
Crit, for a holy priest, doesn't serve to increase your HpS; its function is to increase your HpM by providing SoL procs and Holy Conc procs. SoL, as I've discussed before, actually *lowers* your HpS, but it provides very good mana efficiency. However, the long-standing anti-crit argument is still relevant-- namely, that crit heals are typically mostly over-heal. So long as you have enough crit to keep Holy Conc rolling, you have enough crit.
Intellect is a decent choice. Intellect boosts your regen from spirit, boosts your crit, and boosts your maximum mana. This last is important because Replenishment works as a percentage of base mana, and Replenishment is both widely available and very powerful. However, Intellect is still not terribly strong for a Holy priest.
I'm not a fan of haste. Most healing stats benefit both your HpS and your HpM, but Haste only helps your HpS. Haste provides the most benefit to DPS classes that chain spells together; for healers, unless you're chain-casting, you don't see nearly the same rewards. Sure, it gets your heal off faster, but the amount of haste required to provide a substantial boost to your healing speed is crippling to your other stats. (Note: this is further proof that the Prophecy 3-piece bonus was wicked overpowered... though not as much as 3-piece Transcendence.)
Mana/5 is, by my reckoning, the weakest stat for a priest. Spirit is what we use for our regen, and it has numerous advantages-- racial and talent scaling improvements, multiple talents to boost spirit's contribution, and Blessing of Kings, to name a few. You get much more mileage out of spirit, even post 3.1.
So, the upshot: I would emphasize spirit and spellpower in my gear, followed by intellect and crit, and trailed by haste and mana/5. For gems, I would go with Purified gems in my red slots and half my blue slots, Seer's gems in some blue and yellows, and either Luminous or Potent in my other yellows.
DISCIPLINE PRIESTS
Many of the arguments for Disc priests are similar to Holy, except that intellect is far more important and spirit significantly less. Rapture, which along with Meditation provides Disc with its endurance, is based off of your total pool. Plus, Mental Strength provides Disc priests with exceptional scaling.
Disc priests get considerably less out of spirit, though it remains an important stat to power Meditation and (with Enlightenment) scales well. Crit remains a good secondary stat for Disc because it powers Divine Aegis. Post-patch it is no longer the end-all stat for Disc priests because Divine Aegis was uncoupled from Rapture. Still, crit provides much-needed power for Disc priests.
I still don't particularly like haste. I have Penance. I have strong Shields. I have Borrowed Time. How much faster do you want? If I *really* need haste, I have Power Infusion in my back pocket.
So, for gearing priorities: spellpower, int, and crit is focus areas, followed by spirit, and trailed by mana/5 and haste. For gems, I like to put Brilliant in yellow slots, either Purified or Seer's in blue, and Luminous in red.
SHADOW
I'll admit, I'm kinda fuzzy on shadow gearing. I know spirit is good, but crit is now very tasty and it stands to reason that haste is powerful as well. And you can't forget good ol' spellpower, even though it's no longer the basis of the build's regen capabilities. I don't know.