With the changes to Wrath, the developers have been very transparent as to what they intended for each of the trees.
Disc priests are to be, of course, the most durable of the priests (and they have several tools within the tree such as pain suppression and divine aegis on crit heals to ensure this). Many of their talents focus on increasing the survivability of this cloth-wearing healer. On the other hand, they also intend for disc priests to be amazing single-target healers. Considering their divine aegis talent, I am thinking crit shall play more of a role into how disc priests gear. It is also said that disc priests have been the best healers for tanking Death Knights as of the current Wrath beta build.
Holy priests have been geared more to be area of effect healers. Granted, they are strong all-around healers with potent single target heals in addition to some pretty crazy area of effect heals (circle of healing, which is now a "smart" heal and heals those with the lowest HP out of the targets it can heal). Some mana issues have been addressed, but overall I think you're going to see holy priests mostly sticking to spellpower (the combined spell dmg and healing stat) and spirit.
Shadow priests are still your damage dealers, but I think it shall grow easier for them to find groups though maybe not quite so easy to find raids. In BC, shadow priests were used to "recharge" mana users with Vampiric Touch. Shadow damage they dealt to a target returned a proportion of mana back to the party and the shadow priest. Now, Vampiric Touch has been changed to only return mana back in proportion to damage dealt by a specific spell: Mindblast. Granted, mindblast is a substantial source of damage for shadow priests and is in the damage rotation but eek. It does drop raid utility. On the other hand, hybrid DPS'ers (ret paladins, feral druids, shadow priests, etc) are not being punished for being a hybridized class as opposed to a "pure" DPS class (rogue, hunter, mage, etc). So maybe while shadow priests won't be sought over because they can return such high amounts of mana, Blizzard hopes they'll be brought because of player skill as they should be just as useful as any other DPS class in getting through content.
Another interesting change to shadow is the ability for mind flay to crit. This, in addition to deep shadow talents that increase crit for their spells, means that shadow priests shall be looking to gear for crit as well - a stat that they previously ignored.
We'll see how Wrath treats us!
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