History:
You might have seen a few posts here and there from disgruntled people about how warlock dps is low. You also may have seen people immediately counter with "well I'm #1 in my raid..." or "I'm still top 5.". Allow me to explain:
All warlocks who compete for dps are basically destro right now. If you have any raiding experience from Burning Crusade, you really can't fuck this up. Immolate then faceroll the rest of your keys. There's more to it than this, but its ~very~ straightforward. This is why most people claim to be top dps.
Mages, our primary competition historically, has arcane as its top dps spec. This requires a bit of finesse on the part of the mage, because it relies on gear (more crits), RNG (Missile Barrage procs), and mana management. Bad mages will intentionally lower their dps so that they don't have to deal with the mana issue.
Rogues require a bit of finesse in order to maintain Slice and Dice while managing combo points for Eviscerate. Alternatively, a mutilate spec requires a lot more finesse in how you work your Deadly Poison stacks with Envenom and Mutilate. Bad rogues will lower their dps through bad combo point management or poor use of poison stacks.
Hunters are equally easy to play as warlocks. Low dps tends to mean poor choice of arrows, spec, or less gear than you have.
So basically, the warlocks who are in a raid that are not being out-damaged by the above 3 classes are playing with people who need to try harder. They should be outdamaged. Hybrids are another story altogether. Druids are allowed high dps because of how complex their rotation is (as hard as Affliction in 3.0.2). Unholy DK's and Warriors require more than one target to outdamage pures.
Current (3.2) and why this is:
Scaling. Think about it - in lesser gear, Warlocks win. But in Ulduar, Mages could compete again. In TOC on non-gimmick fights, Mages are ahead. Warlocks and Mages use the exact same gear (basically) and gem similarly. Thus the main difference between the two classes is how they perform as their gear improves. An Arcane Mage has a net Scaling Factor of 6.8592, ignoring hit rating. An Affliction Warlock has a net Scaling Factor of 5.2096. A Destro Warlock has 5.5300.
This means that despite having a higher base damage component from talents, spell coefficients, and base spell damage, Mages eventually will out perform a Warlock because they get more damage from their gear. This is also how two specs interrelate - Destro is better than Affliction, but not by much other than the mechanics of the fights.
I want to point out something - this does not mean a damn thing to blizzard and it shouldn't. There has to be a difference in stat values otherwise whats the point? That's the beauty of design - you get to make two different things with very different numbers and have them fight each other. However, the problem then remains that the mechanics of "Top DPS" specs for all the classes beating Warlocks all favor the kind of fights they have been designing. TOC heavily favors any class with passive aoe (non-channelled) and minimal ramp-up time. Warlocks have both and thus the fights begin in favor of someone else.
This gap with the fight types is now too wide to let go. Blizzard steps in.
3.3 Scaling - Why we no longer need a hug:
There are enough changes, specifically via Glyph of Quick Decay, to warrant a change.
Arcane Mage has changed to 6.7848. -.0744
Affliction has changed to 6.1297. +.9201
Destro has changed to 5.3081. -.2219 (note we gained in Imp crits via Ruin, which do not show up here)
You may note that we did not catch up to Arcane. This is intentional - we gained more dps via pet changes, which are for the most part unaffected by our personal scales as these conversion numbers were not changed in the patch. Also, Arcane does require a bit more thought and skill to play, thus warranting higher dps.
TLDR/Summary: We'll catch up. We won't be #1 in BIS gear, but we've gained enough of an advantage to no longer warrant generic complaints and "mah dps is 2 low" troll posts.
And I'll probably get slammed because you really can't compare scale factors like this, and it's true and extremely inaccurate. However, it's close enough to make a point, and that's where we're coming from and where we're going.