Priest talent analysis

A Priest's Talent trees focus on the Priest's two main roles in a party: Mitigation/Healing (Discipline/Holy respectively), and DPS (Shadow).

Improved Power Word: Shield
2 points: Increases the damage absorbed by your Power Word: Shield by 10%.
 * Essential discipline healer skill. Also makes Glyph of Power Word: Shield more potent.
 * Solo Utility: Normal mobs will rarely hit hard enough to demolish the shield, but if you find yourself dying a lot a boost in the amount of punishment the shield can take is good for making an escape.
 * Raid Utility: No-brainer for discipline priests. Generally not taken by Holy or Shadow priests for build reasons--the benefit offered by the talent just isn't enough to make it viable.
 * PvP Utility: Shields are your lifeline in PvP, so making them 10% more effective might save your life. But remember: it won't be much help if an opponent can dispel your buffs.
 * Bottom Line: Essential for discipline priests. Great for PvP. Holy or Shadow PvE can skip this.

Twin Disciplines
3 points: Increases your Shadow and Holy spell damage and healing by 6%.

Mental Agility
3 points: Reduces the mana cost of your instant cast spells by 10%.
 * Good for the people who like to cast instant spells. However, if you don't tend to have mana problems, there's really no need to get it.
 * Solo Utility: You use less mana on your instant cast spells, so you have both more mana for the longer cast times and slightly less downtime.
 * Raid Utility: This talent is a mainstay of any Discipline (Power Word: Shield) priest. Holy priests will want this, especially those who prefer the Renew Chakra, but will have to choose between this and other equally useful talents. Shadow Word: Pain, Shadow Word: Death and Devouring Plague will be the main shadow spells affected. The DoTs are very high mana cost, though SW:P is refreshed automatically with talents.
 * PvP Utility: You'll be expected to use a lot of PW:S in PvP, so another useful talent. However, the other talents in this tier are probably more useful.
 * Bottom Line: Discipline? You want it. Holy, Shadow or PvP? Take it only if you get mana problems.