Intellect is the most important stat for a Destruction Warlock as it increases the damage of your spells, this is why Destruction Warlocks just want as high item level gear as possible.
Haste decreases the cast speed of you spells. It also buffs Immolate as the tick rate increases and the cooldown of Conflagrate is decreased. The more haste you have, the better Soul Shard regeneration you will have. Haste also scales with your demons.
Critical Strike increases your chance to critically hit with all spells and abilities. Chaos Bolt always ciritically hits and its damage is increased by your ciritical Strike. Immolate criticals also have double chance to produce a Soul Shard.
Mastery: Chaotic Energies increases your damage dealt by a random amount as well as reducing damage taken by a random amount. Its between 0 and the maximum amount which dpeends on your mastery rating.
Versatility is just a flat damage increased and damage reduction, in which your demons scale with too.
Kil'jaeden's Burning Wish takes up a valuable trinket slot but sports a high amount of passive stats and a useful on use burst effect.
T19 bonuses are really strong for Warlocks
T19 2 piece is worth losing 10-15 ilvl on items for, it is even strong in cleave situations as it helps get more Chaos Bolts out.
T19 4 piece is worth losing 40+ ilvl on items for, it is a 10% DPS increase, try and get your hands on this ASAP.
A general rule for warlocks if its 10 ilvl upgrade ALWAYS equip it, as ilvl is everything for a destruction warlock.
The main 4 trinkets in which you want to try and get your hands on are as follows
- Whispers in the Dark (Gul'dan)
- Erratic Metronome (Chronomatic Anomaly)
- Brinewater Slime in a Bottle with Haste or Crit (Helya)
- Chrono Shard or any other int/haste trinket from Mythic+
Races are a lot less beneficial than they use to be however the 'best' ones for DPS are;
Socket Quick Dawnlight in all.
Flask: Flask of the Whispered Pact
Food: Azshari Salad