Classic Warlock DPS Stat Priority

1. Spell Power (or +Shadow Damage);
2. Spell Crit Chance;
3. Hit Chance until 16%;
4. Spell Penetration
5. Intellect;
6. Stamina;
7. Spirit.

While you can follow the basic priority above, it is important to understand how the stats are measured. As you start to gain more Spell Power and Critical Strike chance on your gear, your Shadow Bolt casts will hit harder. This means that each miss will cost you more damage, so Hit Chance will go up in value as you get more of the other stats. Similarly, if all of your spells are hitting, Critical Strike chance becomes more valuable since there are more potential critical strikes. For optimal DPS, it is best to consult a simulation spreadsheet of some sort to see how your character's stats are currently sitting, but in general you can follow the above priority early on.

Spell Power increases the damage done by your spells by a flat amount per point, with a different coefficient per spell. Your most common spell, Shadow Bolt, has a coefficient of 85.7% along with a base damage of 510 (at max rank). This means that to calculate the damage of a ShadowBolt cast, you just do 510 + (Spell Power * 0.857) = Base damage of one Shadow Bolt. Spell Power is an incredibly strong stat, and Warlocks scale quite well with it. In general you should be looking to have Spell Power on almost every piece of gear.

Normally spells will critically hit for 150% of their normal damage. With the Ruin Destruction talent, Shadow Bolt and your other Destruction spells will critically hit for 200% of their normal damage instead. This makes Critical Strike an incredibly valuable stat for Warlocks that scales quite well since one Critical Strike more than doubles a ShadowBolt's damage.

Raid bosses are treated as being level 63, meaning they will be 3 levels above you. Against enemies 3 levels above you, you will have a base 16% chance to miss with your spells. Hit chance can improve this, up to 16% at which point you will never miss spells against raid bosses. Hit chance is a huge damage gain, but not critical enough to stack early on. You should take it where you can, but you will probably not be able to get your 16% hit cap until much later on.

In WoW Classic, bosses have varying amounts of resistances depending on the type of boss you are fighting. A boss in Molten Core is likely to have very high fire resistance, which is why most Mages will play Frost instead of Fire initially. As a Warlock, you should pay attention to which bosses have higher Shadow resistance. Shadow Bolt is a spell that can be partially resisted, so if an enemy has 25 shadow resistance, that converts to 6.25%, which means 6.25% of each ShadowBolt cast will be resisted. Thanks to Curse of Shadow, bosses will have 75 less shadow resistance by default, but any resistance beyond that can be reduced by spell penetration.

Each point of Intellect gives you 15 max Mana and 0.0165% chance to critically strike. Critical Strike is very strong for Warlocks, but Intellect is an inefficient means of getting it since it takes roughly 60 Intellect to get 1% Critical Strike chance. The max Mana is also nice, however, since you will run out of Mana during some encounters and be forced to cast Life Tap to regain your Mana. Having a higher Mana pool means more Shadow Bolt casts before you need to stop. This is your best primary stat, but is still a much lower priority than the other secondary stats above it.

Each point of Stamina gives 10 max health. Having a reasonable health pool is important to surviving AoE damage and other raid mechanics in WoW Classic, and while not worth going out of your way for, is important to have a reasonable amount of. This will mostly just be found passively on your gear anyways, and is not worth worrying about.

Spirit increases your out of combat health and Mana regeneration, but you can get the Mana regeneration in combat by following the 5-second rule. The 5-second rule is that you will begin regenerating Mana when you do not spend any Mana for 5 seconds. This is pretty useless for Warlocks since they use Life Tap as their primary means of regaining Mana, but if you choose to wait and use Life Tap repeatedly at once, you can also regenerate some Mana passively since Life Tap technically does not cost Mana, and will not count against the 5-second rule. This is still a pretty useless stat for Warlocks outside of leveling, though, and should be avoided.