Gear selection is a contentious issue for many warlocks. Hell, if you've ever debased yourself by visiting the official forums you should know that for many people, gear selection is tantamount to one's worth as a person. Rubes of the official forums aside, though, there is more than one school of thought about how to select gear. Some of them are absolutely terrible (we call these "Stamina Stackers") while some of them have a legitimate case to make. Since my goal with this blog has absolutely nothing to do with objectivity, what follows is an approximation of how I select my own gear. It seems to have worked pretty well so far.
As a forward to my discussion on gear selection, I want to reiterate--for those who have never read my blog before, or those with a poor memory--that I'm talking about PvE affliction gear here. Usually I go ahead and leave that unstated, since it's implicit in the way I talk about things. However, when discussing gear that distinction is nowhere near as clear--and far more important. PVP warlocks, PvE demonology warlocks, and PvE destruction warlocks have different priorities for each of their stats--even if we all use the same ones.
I'd also like to say, before I start, that I could have simply written about specific pieces of gear I think a person should get, but that wouldn't really be helpful in the long run. You might say that rather than giving you a gear-fish, I'd like to teach you how to catch a gear-fish. Gear-fishing, however, is no simple matter; there's MATHZ involved. And while a good WoW player should never be too lazy to play with the numbers, I think there's a threshold where it becomes acceptable to approximate and move on. I call that threshold the "damn, where the fuck is my old calculus textbook" line.
Having said all of that, lets start to break this down. Selecting gear is, essentially, a choice between various stats. There are seven stats a warlock should care about, and every other stat is absolutely and utterly pointless for a PvE affliction warlock, and should be completely ignored. Each of those seven stats serves a specific purpose, and should be weighted according to the importance of that purpose.
Spell Hit is of paramount importance--at least until you get 446 of it. This is a pretty standard requisite for all the DPS classes but frequently I hear some young warlock proclaim something akin to "I never miss anyway, so I don't need spell hit." To which I must respond that anybody who thinks they never miss isn't watching their combat log closely enough (May I recomend Scrolling Combat Text Damage?)
Consider this: haunt is the groundwork for any affliction rotation, and it has a nice big 8 second cooldown on it. If haunt were to miss, then that's at least 8 full seconds of unbuffed dot ticks--a massive blow to any warlock's standing on the DPS meter.
Stacking hit isn't very fun, though. In fact, it's pretty boring to stack compared to things like crit. Fortunately, there is relief if you spec correctly. While it is true that without talents, you must get 446 hit rating to be hit-capped, if you put even 1 point each into Suppression and Cataclysm (in the affliction and destruction trees respectively) that will bring your requisite hit rating down to 420 to be hit capped for all of your spells. If you put TWO points in each, the hit-cap is lowered still further to 394! And if you're able to put a total of six points aside for putting 3 in each of the two talents, then the hit cap goes all the way down to 368. Obviously those talent points might be better spent elsewhere, but they are a nice cushion that can allow you to play a bit above your gear level.
Spell Power is a warlock's bread and butter. There really isn't too much more that can be said about it. Gearing up is largely the quest to make this number higher and higher without getting un-hit capped.
Spirit was, until very recently, utterly useless. However, in patch 3.0.2, things changed! Not only does Life Tap scale with this stat, but (far more importantly) 30% of our spirit is turned into Spell Power by our Fel Armor! This isn't horribly significant, but it is worth keeping in mind if you're ever faced with a tough decision. Lets say, for example, that you're trying to choose between two pieces of gear. One has 30 spell power, and the other has 100 spirit and 10 crit rating. Now, spell power is always preferable to crit rating for an affliction warlock, but, that 100 spirit will be turned into 30 spell power by fel armor, meaning that the second piece of gear effectively has 30 spell power and 10 crit--a clear winner when compared against the first piece, which has only 30 spell power.
Crit Rating is everybody's favorite stat, and thanks to Pandemic, now affliction warlocks can play with it too! That doesn't mean we're allowed go go wild, however. But thankfully, most pieces of endgame gear have at least two green stats (Spell Power, Hit Rating, Haste Rating, or Crit Rating) and once one has gathered enough hit rating, it's a perfectly legitimate strategy to use mostly Spell Power / Crit Rating gear. In fact, that's precisely what I do.
Intellect is to crit rating what spirit is to spell power...sort of. Intellect has two primary uses. First, more intellect means deeper mana pools, which means less life tapping and more pew-pewing. More relevant to the aforementioned relationship however is that intellect raises crit rating. To be precise about it, 167 intellect translates into 46 crit rating at level 80. 46 crit rating being the requisite amount for a full 1% crit chance. So, much like the example I used with spirit above, enough intellect can make up for losing some crit rating on a new piece of gear--and in fact, it's preferable to get your crit chance off of intellect, since intellect also increases your mana pool.
Haste, I will be honest, is a bit of an enigma to me. Rather than try to sound like I know what I'm talking about when I don't, I'll say this: it makes your spells go faster, this is awesome. However, in most cases it seems like you must choose between gear with spellpower/haste, or spellpower/crit. Which either means accepting a very low crit chance, or balancing out the two stats and having them both be mediocre. Personally, I stack crit rating. This may well not be the best choice, as I have seen spreadsheets that imply haste to be the superior stat. I am slowly building up my haste, and hope to get a better idea of just how much it helps my DPS in the future.
Stamina happens. That's really all there is to say. Yes, stamina is an important stat for warlocks, it keeps us alive and it allows us to do mad life-tapz...but it doesn't need to be considered. Sentai's unbuffed HP has fluctuated from 14,000 to 18,000 and back again, and it doesn't matter. All gear has loads of stamina on it these days, and a piece of gear would have to be an absolutely titanic loss of stamina in order to warrant even a passing consideration in the decision making process. In honesty, the only reason I even included it in this list is because there are PVE affliction warlocks out there, the bad ones who showed up to Kara in Gladiator gear, who think stamina is the absolute most important stat for warlocks. It is not. At all.
And that's the four-one-one on warlock gear selection. The most important thing, of course, is balance. 446 hit rating is worthless if you've got 100 spell power, and by the time you've got 1800 spell power you should probably have at least 15% crit, or a nice bit of haste if you decided to go that rout. I'd like to thank my warlock-buddy Kolrawn for helping me work out the best way to break this all down, and I hope that it ended up being helpful.
A closing note on spell penetration: It's essentially useless. Here's what WoW Wiki says on the matter:
PvE, targets of a higher level than the caster have a base resistance (2% of mitigation per level difference) which cannot be overcome by spell penetration nor by any other stat. Otherwise, very few mobs and raid-level bosses have a substantial amount of resistance to overcome.