The Discipline tree has gone through great changes with Wrath of the Lich King. While it was seen as a PvP spec in Burning Crusade it has now become a competitive healing tree with some unique abilities. One of those signature abilities the shield. While any Priest is able to cast Power Word: Shield a Discipline Priest's shield is much stronger and able to reflect damage upon the attacker if certain talents are selected. Additionally the huge mana cost of the shield spell are greatly reduced for a Discipline Priest. They also get a talented group version of the shield spell.
Priests of the Discipline tree also get some special buffs they can cast on other group or raid members. One of those is Pain Suppression, reducing the incoming damage a target takes but also reducing its threat. Even with this downside a skilled Discipline Priest will be able to use this spell as a much needed tank saver. This spell becomes necessary in fight like Sartharion with 3 drakes, where the fire breath of Sartharion will kill the tank unless a cooldown like Pain Suppression is used. Power Infusion boost a player's damage or healing output by greatly increasing their attack- or casting-speed for a short period of time - a mini Bloodlust if you will.
Naturally the Discipline Priest does not spend all his talent points in one tree, but also invests into some basic talents within the Holy tree. The ultimate Discipline talent is a must have though, so he will have no more than 20 talent points to invest into the Holy tree.
The Discipline talent tree has lost nothing of its power it had during Burning Crusade and still makes the Priest a hard to kill target in PvP combat. His fast Mana Burn and Mass Dispel make him an enemy to be afraid of.