Destruction Warlock
Rotation
Opener: Curse of Doom on a boss (or Agony on a trash mob), Corruption, Immolate, Shadow Bolt.
Priority: Shadow Bolt with Ruin is the whole spec; keep Immolate up only if you have Improved Immolate; Conflagrate when it is up; Shadowburn to finish. Life Tap for mana.
Cooldowns: Shadowburn as the execute; Soul Shatter is not in Classic's kit, so watch threat and let the tank build it.
Forever notes: The raid DPS tree. The Nov 2 damage pass raised Shadow Bolt; threat is your enemy, so a Tranquil Air totem or a threat-drop plan is part of the rotation.
Stats
Spell Hit to 16%, Fire and Shadow spell power, Spell Crit (Ruin doubles crit damage), Intellect, then Stamina.
Ruin makes crit the best stat after hit; chase it on every slot.
Numbers for every spec and a gear comparer: stat weights.
Where it ranks
See the tier lists for PvE and PvP placement, and Classic vs Forever changes for what Blizzard altered in this class.
Wowhead and Icy Veins keep spec guides of their own: Icy Veins Destruction Warlock, Wowhead class guides. Our rotation notes are editorial, written for the Nov 2 tuning.