Attributes And Their Value
There are many attributes in this game to keep track of, and determining how to value something can be very difficult. To further complicate things, certain attributes are much more useful to one talent spec than they are to another. In this chapter, I will explain the various attributes and their worth to a warlock as best I can, to help you figure out which is best to get, and when to get it.
Before covering the basic attributes, let me deal with Armor Class on items:
Warlocks can only wear cloth, and cloth has a very low armor class (AC). When deciding if you want to use an item, you may as well leave the AC on it out of the consideration. If your armor breaks while soloing, you can always head to the nearest merchant and repair it. Thankfully, cloth is the cheapest armor type to repair.
Base Attributes
In the early game, you do not often find anything special on items beyond the base attributes. The order of importance for the base attributes to look for as a warlock can be debated, but is generally accepted to be:
Stamina > Intellect > Spirit > Agility > Strength
You may occassionally find an item with spell damage on it early on, which is quite alright to use. You wont truly be focusing on stats like that until the late 50's, early 60's, but it is certainly not a bad idea to get some spell damage for yourself earlier on. Before I explain spell damage and beyond, let me cover the base stats and their use to the warlock:

Stamina: Putting stamina first might not seem like an obvious choice as primary statistic to a beginning warlock, but there is a simple reason for this: We can trade our life out for mana using lifetap (With a 20% bonus with the improved lifetap talent), but we cannot trade our mana back for life nearly as quick or efficiently. Seeing that running out of hitpoints will kill you, and running out of mana does not, the reason for stamina as a primary stat becomes clear. You will be much more resilient while soloing

and grouping, have better chances in PvP and live through painful AE’s during raids longer. Having a smaller manapool as a result does not harm you as much, as you can trade hitpoints for mana whenever you need to.
I’ve seen some warlocks that choose not to use lifetap, which is a bad idea. If you have a full healthbar, and you can use some mana, there is no reason not to use it. You will benefit from both health and mana regeneration. (while not in combat) Furthermore, you can use bandages, drain life and siphon life (If you have the talent for it) to gain life back faster after lifetapping.
Intellect is second. You do need some mana to work with, and beside that, intellect increases the chance that your spells will land a critical hit by 0.01% per point. So 100 intellect will give you +1% chance to land a critical spell. I was a little underwhelmed by such a small improvement. It is nice to have a lot of intellect for the increased manapool and extra crit chance, but it wont help you last longer when you end up taking damage. This is not as important if all you do is solo or group, but for PvP you will need the extra hitpoints.
Spirit comes third, after intellect, because your health does not regen in combat. Furthermore, your mana does not regen for 5 seconds after casting a spell, making the spirit stat only of any real use when you are not fighting. And while not fighting, you can use food and drink, which is a lot faster then any amount of spirit you might possess. Spirit is uninteresting to a warlock.
Agility comes third because its main positive effect on you, a bonus to your dodge chance, is small. It also gives you an increased chance to land a critical hit in melee, but if you melee enough to warrant focusing on agility, you might try making a melee char instead of a warlock. ;)
Strength is last. It’s for melee characters, and you don’t need it. Sure, you can put out a little melee damage with that dagger or staff, but looking for strength in gear as a warlock is not a good idea.

Spell damage
As you proceed into the higher levels, spell damage becomes more prominent on items. This is a good thing, because it is a key statistic. Spell damage affects all of your damage spells as well as health funnel.
There are two ways in which improved spell damage can be shown on an item:
1. Increases damage done by Shadow spells and effects by up to 10.
2. +10 Shadow Damage
The first option is the one you will most regularly see, the second is reserved for items with the random modifier 'Of Shadow Wrath'. Both options do the exact same thing, only the wording is different.
The effectiveness of +spell damage depends on the rank of the spell you use, the casting time, and what type of spell you are casting.
A longer casting time results in a greater bonus from spell damage, and you are best off using the highest rank at your disposal.

Spell Damage Bonus Co雈ficient
Fancy words for saying how much of an effect you get out of your spell damage. Each spell has a different co雈ficient, determined in part by the factors I already mentioned above, and by a few more I won't bore you with.
Here's the data:

All of the numbers presume level 70 and using the max rank of the spell available.
Let me give an example: A warlock with +500 spell damage casts Siphon Life. Siphon deals a base 630 damage over 30 seconds without factoring in talents. Siphon receives a 100% spell damage bonus co雈ficient, which results in a flat 500 damage increase from your spell damage. Siphon does 630 + 500 = 1130 damage over 30 seconds.

Another one; the same warlock with +500 spell damage casts Immolate.
Immolate does 327 DD and a 615 DoT. The direct damage portion receives a 20% bonus from your spell dmg. The DoT portion receives 65%:
327 + (500 * 0.2) = 427 DD
615 + (500 * 0.65) = 940 DD
That pretty much covers the effect of spell damage, I hope it wasn't too confusing!
As a sidenote; spell damage has no effect on wand damage.
Spell Resists, +Spell Hit and Spell Penetration
Let me start off with resists. An actual spell resist is caused by your target having resistance against that damage type. This can result in dealing only partial damage or a complete resist. You can reduce resists caused by resistance with Curse of Shadows and Elements as well as with items with Spell Penetration. Spell Penetration simply lowers your target's resistance to your spells. You wont find many regular mobs with any resistance whatsoever before Outland.
That wraps up spell resistances, onwards to spell hit.
You may be asking yourself, if most mobs do not even have spell resistance, then why am I still seeing spell resists? This is because your spells can actually miss. The combat log will report these misses as if they were a resist, while in fact they are not.
Your chance to hit with a spell is dependant on the level of your target relative to your own.
–You have a 96% chance to land a spell on a target of your own level.
–95% if the target is a level above you.
–94% if the target is two levels above you.
–83% if the target is three levels above you. (87% if the target is another player)
–72% if the target is four levels above you. (80% if the target is another player)
–61% if the target is five levels above you. (73% if the target is another player)
Your miss chance can be reduced no lower than 1%, so having 3% spell hit is sufficient against equal level mobs.

Later on, should you venture into raids frequently, you may desire a higher +hit rate. Most bosses are three levels higher than you, and you have a base 17% chance to miss there, meaning you would need 16% spell hit to reduce spell hit to its minimum of 1%.
Obtaining such a high hit rate from gear alone would be foolish though, as you would be giving up huge amounts of damage gear. You have to decide where its worth giving up other stats for more spell hit, and where it isnt.
Lastly, the Supression talent in the affliction tree counts as spell hit. It can give you up to 10% spell hit for your affliction spells. You would only need 6% spell hit from items to reach 16%.
Spell Crits
Many items increase the chance for your spells to crit. Its value to you is very dependant on your talent spec. An affliction lock has much less use for spell crits than a demonology lock, and a destro lock favors crit more than any other spec.
Note that the value of spell crit doubles when you have the ruin talent, so that is a major factor in determining how useful the stat is to you.
The Shadow Vulnerability talent benefits from crits as well, but not to such an extent that an Affliction lock with that talent should start gathering spell crit in favor of damage.
Spell Haste
This is a newer attritube, it didnt exist prior to the Burning Crusade's release. It reduces the cast time of your spells, as well as reducing the duration of your channeled spells (while keeping the total damage done the same). It does nothing for instant spells or DoT duration.
Resilience
This attribute reduces the chance for you to be criticilly struck by an attack, as well as reducing the damage you take from the crits you do take. It is primarily a PvP stat.

Rating Conversion Table
Items have ratings, but you generally want to know percentages. The actual percentage increases depend on your level. The lower your level, the higher the percentage received. See below for the level 60 and 70 conversions;