PvE Shadow Priest
Thanks to WoW Patch 1.7, the PvE shadow priest is now a valid build. A PvE Shadow Priest can successfully deal tremendous amounts of damage. In fact, end game a Shadow Priest have the potential to out-damage all other classes.
The most important element to the shadow priest is his gear. Shadow Priests normally cannot deal enough damage to take down elite mobs before running out of mana, and therefore is not useful in instances. However, since the shadow priests damage is augmented by three talents (darkness, shadow weaving, and shadow form) each point of shadow damage is actually a 1.32 point increase.  Thus, a shadow priest with a good bit of shadow gear can deal exceptional damage.

A PvE specced Shadow Priest exchanges Unbreakable Will for Silent Resolve and places several points in Shadow Affinity. My research has dictated that 35% is the optimum threat reduction, but more experimentation is needed. Mental Agility is sacrificed for more points in shadow.

There are large disadvantages to this build. First, this build is built around maximizing damage and controlling agro.  Therefore the priest lacks silence ( a shadow priest’s most powerful Player versus Player ability), does not have improved resistance to fear, or the casting efficiencies gleaned from the discipline tree.   They are also the weakest priests at healing, and does not excel at Player versus Player combat like a Combat Shadow Priest.

Finally, they are completely gear dependent. Because the shadow spells deal medium damage, but have a large multiplier, the PvE Shadow Priest must have a significant amount of shadow damage added to their equipment. A minimum shadow damage of 150 is suggested. At about 200 shadow damage, the shadow priest can out damage warlocks in instances. At 300 shadow damage, the priest begins to out damage even well equipped rogues in Molten Core. Unfortunately, the shadow priest gives up a great deal for his shadow damage.  The plus to damage comes at the cost of spirit and intelligence, leaving them without the ability to last in long battles like other high damage classes.