Vanilla Shadow Priest know-how

The key to winning as a shadow priest in World of Warcraft pvp is to think like a tank instead of like a spellcaster. This is often a diffcult change of approach, especially for players who are used to playing their spellcaster as a "squishy" class, and who prefer to completely avoid close range combat.
Shadow Priests are an entirely different class and require a significant change in strategies in order to be successful. Shadow Priests lack the enormous crits, high-damage spells or mana efficiency of the traditional ranged spellcasting classes like mages, shamans and druids, and they lack the versatility of warlocks because they don't have pets or a large variety of spells.
What Shadow Priests lack in these areas they make up for in sheer stubbornness. Shadow Priests are without doubt the toughest spellcasting class for melee attackers to defeat, and they are a colossal headache for other spellcasters because of their ability to silence and mitigate damage.
There are three primary strengths around which players should build their shadow priest pvp strategies.
Shadow Priests have the highest physical damage mitigation of any spellcasting class. The combination of shadowform's damage reduction, the armor bonus from inner fire and the ability to raise a shield every 15 seconds makes it very unlikely that a melee attacker will accumulate adequate damage before their health is consumed by a shadow priest's damage over time spells and mind flay.
Using this class feature in combat against melee classes is simple. Don't run. In fact, don't move at all except to change directions for mind flay. As a shadow priest, you are gambling that shadow word: pain, devouring plague, mind flay and vampiric touch will deplete your opponent's health before they can break through your armor, armor bonuses and shields and outrun vampiric touch with their dps. A well-timed psychic scream, especially just before a mortal strike, eviscerate or windfury will often close the door on a shadow priest loss.
The second strength of the shadow priest is that shadow word: pain is the best damage over time spell in the game. Shadow word: pain has the highest per tick base damage and therefore benefits the most from shadow bonus damage. Unlike warlock damage over time spells, shadow word: pain can also cause a stun through the blackout talent, and in fact, rank one of this spell is often used as a ranged stun against running opponents such as a flag carrier in the Warsong Gulch PVP Battleground.
The third and probably most overlooked strength of the shadow priest in pvp is the ability to heal. While vampiric touch is indeed a nice ability, many shadow priest players overlook the importance of dropping shadowform when necessary. Shadowform is primarily an offensive class ability, and as such, should be viewed as optional from the standpoint of a pvp battle where health is becoming an issue, especially against a class with significant burst damage capability like a mage. Formidable is the shadow priest who lands a greater heal mid-battle against a class that cannot heal at all, especially if it crits.
The general philosophy of the shadow priest is one of constant attack. Shadow priests do not avoid, they do not run and they do not concern themselves with numbers of opponents. Compared to other spellcasters like warlocks, mages, druids and other priests, shadow priests are like blocks of solid iron to a melee opponent and very confusing to other spellcasters.
One of the best openings for a shadow priest is mind flay. As strange as it may sound, landing a mind flay at maximum range gives a shadow priest a variety of options. If the opponent chooses to close, they are walking right into a psychic scream and the loss of their trinket. If they choose to run, the shadow priest can simply allow shadow word: pain spells to tick while they try to stay in mind flay range. If the opponent answers with ranged attacks of their own, the shadow priest can simply answer with vampiric touch and give their opponent a large damage penalty while winding up a mind blast.
One important key to keep in mind while fighting in shadowform is that shadow word: pain, especially at maximum ranks is relatively expensive to cast at several hundred mana. Mind flay is far more versatile and far less expensive mana-wise, even though it does less damage. Shadow priests are notoriously mana-inefficient, and so must do what they can to preserve as much mana as possible by employing such strategies as downranking psychic scream and possibly putting a point in vampiric touch at higher levels.
Shadow priests have always been an extraordinarily powerful class and build for World of Warcraft PVP. The class is very simple and the bonuses and abilities in the shadow tree compliment each other in very powerful ways. The class is somewhat limited by a "come as you are" itemization system. The only true weakness the shadow priest suffers is the lack of efficient mana regeneration. However, with full mana, there is no class or build in World of Warcraft that can count a shadow priest as an automatic win.