"Woe to you, affliction warlock! For your dualistic lot in life is one both of glory and of shame; for while you may top the meters on Patchwerk, never will your name be seen above rank the tenth when there is trash to clear. And so will you remain in obscurity, your true skill remaining hidden due to the lies told by 'overall damage' reports."
~Anonymous Raid Leader
I'm not one to complain about the weaknesses of my class, or the strengths of other classes. I may have suggestions for improving things, but I really do hate to bitch. That said, affliction warlocks have it bad right now as far as trash DPS goes, which in and of itself isn't a bad thing. Trash mobs are just what their name proclaims them to be--trash. If a group can't manage to tear through trash mobs with relative ease, then they really have no business attempting the bosses that come after said trash.
The problem with affliction's low trash-mob-damage comes not from any actual problems with the in-game situation, but rather, from the damage meter, and the perception it creates in peoples minds. Now don't get me wrong--damage meters are wonderful, amazing tools. If not for damage meters, I never would have found out that I was consistently one of the worst damage dealers in any group, which means I never would have sought help and never would have become the symbol of mediocrity that I have become today. Furthermore, without damage meters I would have no way to test my new rotations and specs. And hell, without damage meters, I wouldn't be able to playfully compete with my fellow DPSers for the top DPS spot.
The problem with the damage meter is that most people don't know how to use it properly. Almost everyone keeps the thing stuck on 'overall damage,' which in my experience both as a DPSer and a raid leader, doesn't do anybody any good. Far too often this method of looking at the numbers allows a small group of people to climb to the top of the meter doing trivial things such as slaughtering the trash mobs or spamming AoE on nonelites, while their actual contribution in difficult situations is frighteningly lackluster.
Furthermore, as a motivating factor, damage meters become meaningless when only overall data is used. Invariably, in any instance, a small group of people quickly establish themselves as the 'top' DPS, while other players are left in the dust. Given a boss or two, the damage meters will settle into a rather stable arrangement, that won't be changed even if the people at the top stop trying, and the people at the bottom use every trick they've got in an attempt to pull ahead.
Personally, I find the most accurate way to view the data is to set your damage meter to only show the numbers for the current fight. This way, everybody has a clean state at the start of every single pull. When I've been doing well throughout the instance thus far, this forces me to 'keep it up,' in order to maintain my lead. Similarly, when I've been doing poorly, I am less likely to get discouraged, and more likely to try harder to improve my standing on the next pull.
All of that aside, I am most firmly of the opinion that even though affliction warlocks can't be expected to do good DPS on trash, that doesn't mean that we shouldn't try. To that end, I've begun to play with a variety of alternate tactics for trash mobs, and while I've yet to achieve any truly great level of success, I have managed to put forth a decidedly respectable showing on individual trash pulls now and again. Once or twice even managing to get the top spot! But I'm getting ahead of myself.
Before you really begin to think about how to do good damage on trash mobs, you must try to understand why it is that affliction has so much trouble with them in the first place. Affliction Warlocks are sometimes called "DOT locks," after their primary form of attack. DOT, as everyone knows, is the acronym for Damage Over Time. And that, there, is the root of the problem. Time. Affliction warlocks require some measure of time before they can really begin to do damage.
I've done extensive testing of affliction damage using things like real time graphs, as well as friends who agreed to serve as a control group of sorts. What I've noticed is that even several seconds into my rotation, my damage output is altogether pitiful when I'm using my full casting sequence. Then, about 25-35 seconds into combat, my numbers suddenly shoot up at an astronomical speed, as all my dot ticks start going off in force. Before this, however, my numbers would shame a level 65 elemental shaman.
At this point, the issue should be clear. When fighting trash mobs, each target is only going to be alive for a scant few seconds, meaning that the 'shooting up' part of my damage never happens. Ergo, it would seem reasonable to conclude that the only way to really do damage against such mobs is to either significantly alter my rotation, or respec--something I'm not willing to do when my damage output in more important situations is phenomenal.
Altering the rotation, however, is a tricky business, because it either requires memorizing every trash mob in every instance, or it requires tactical senses good enough to analyse a target mob's life expectancy after a glance at it's HP, and formulate a viable tactic based on that. Given the near impossibility of the former, I fear I am stuck with attempting the latter, even given my lackluster tactical skills.
To make things simpler, I've devised a hand full of rotations for different situations. And, whenever I find myself faced with trash pulls, I try and approximate which one of these rotations might work best for that particular pull.
Boom Theory: Seed of Corruption, Rain of Fire, and a Shadowflame now and again for good measure. If you've got a good tank, then this is by far the easiest way to go about things in larger pulls. Not just pally tanks anymore either! I've used this tactic to great success with both warrior and death knight tanks*. Be careful though, if you pull aggro you're very likely to end up dead, and if you piss the tank off, you're very likely to end up looking at a window that says "You are not in the instance group, you will be ported to [Insert Hearth Location here] in X amount of seconds."
(*No offense is intended to my feral druid buddies--I just haven't run with one since Karazhan, so I can't speak from experience.)
The Resilient Trash Mob: Some trash pulls are almost like mini-bosses in terms of how much HP they have, but that still doesn't mean you're going to be able to get your full rotation off. So I abbreviate it: Curse of Elements, Haunt, Corruption, Followed by Shadow Bolt spam, and a re-cast of Haunt whenever it's up. After the first 7 seconds of combat or so, all your casting time will be spent on direct damage casting, so you won't be losing any global cooldowns on dots that won't finish. I sometimes modify this to include Unstable Affliction as well, if the mobs are particularly hearty.
Humanoids Got No HP Humanoid trash mobs last only barely longer than nonelites do in most instances. For these, the AoE approach is by far the preferable one, but if there's only one humanoid left, or if your tank isn't comfortable with the threat created by your AoE, then your best bet is to just sit there spamming the Shadow Bolt button. These mobs will likely go down so fast that even Curse of Elements will be a waste of valuable shadow bolting time. I've considered trying to use Searing Pain on these mobs, given its shorter cast time, but the significantly lower amount of damage it does, and the significantly higher amount of threat that it pulls, makes me dubious that it will be more successful than straight up SB spam.
I've found that, in a general sense anyway, one of these three approaches will almost always serve well enough to get the job done, and keeping available tactics down to three also helps avoid confusion. I think it was Bruce Lee who said that a person shouldn't try and learn too many ways to react to a single situation, because if they do, then when they're in that situation they'll find themselves paralyzed with indecision about which way to react. Something to that affect anyway.
So that's how I deal with trash DPS as a level 80 affliction warlock. It's not great, it's not even 'very good' if I'm trying to out DPS a hunter, rogue, or kitty druid. However, the results I've got from it are certainly respectable enough that I can rest assured my performance on the boss will be noticed, and my actual skill--good, bad, or mediocre--can be judged at least somewhat accurately by the rest of the group.