The funky way my character portrait, target window, and raid information are displayed can be attributed to X Perl, a classic unit frames addon. There are more uses for X Perl than I could possibly make use of, much less cover here. What I do use it for is to display more information in less space. For example, you'll notice that rather than my buffs being in the upper right, they're right under my face. What's more, I can see all my party's buffs, and the target of every person in my party, as well as the casting bars of everyone in my party. It also gives me a lot of customization in how my debuffs are displayed--which was immensely helpful before I started using Dot Timers. More on those later.
One thing about having so much information on my screen at once is that, while it is extremely useful, it can be overwhelming if I need to access a specific piece of information in a split second. To take care of that, I use ArcHUD2, which is the reason for those half-circles on either side of my character. From left to right, starting with the red bar, is my Target's health, my health, then the health of my pet in the small crescent, and on the other side of me my pet's mana, then a casting bar which appears whenever I cast, then my mana. For me, ArcHUD is for my eyes what the home-row on a keyboard is for my fingers. It's the default position my eyes return to after they glance somewhere else for some tidbit of info. [EDIT: I messed up what some of the ArcHUD bars do. They're fixed now.]
One of the most difficult thing about playing the game, for me anyway, is figuring out what abilities do what, and how well they do it. I throw up a DoT, and I'm left to gauge it's effectiveness based roughly on how fast a monster dies. I can look at my combat log if I like, but the combat log is a giant mess of numbers representing dozens of different things. Scrolling Combat Text and SCT Damage keep me in-the-know. Those two addons are responsible for what most people dub the 'clutter' on my screen. And perhaps it IS a little cluttered -- but I always know where to find any information I need, so it works for me anyway. To break it down, scrolling downward to the left of my character is the damage I'm dealing, to the right is a display of things like healing and mana replenishment, and scrolling upward above my head is a representation of all the damage I'm taking. Using these, I can see in real-time what the difference is between Corruption, Corruption with Haunt, Corruption with Curse of Elements, and Corruption with both Haunt and Curse of Elements. Extremely useful for working out your DPS.
While in the past, I refused to use addons to help me time my dots, affliction eventually reached a level of complexity that forced my hand. And honestly, I'm happy it did. The depth of the current rotation requires a quick tactical sense and a good sense of rhythm in order to make the most of it, and DoT Timer just looks plain sexy in the bottom center of my screen. Dot Timer shows countdowns for all over-time effects on a targeted creature, and moves effects further toward the bottom of the list depending on how little time they have left compared to the other dots. It goes a long way toward helping a player create that all-important rhythm that goes with playing affliction these days. And as an added bonus, it keeps track of your cooldowns!
It has always pestered me that Blizzard gives some classes more abilities than can possibly fit on an action bar. Most people are fine with this. They simply don't put their useless abilities on action bars, keeping only the most essential buttons easily accessible. I, however, am certain that one of these days I'm going to suddenly be in some sort of situation where Curse of Weakness is useful, and I don't want miss the moment while I'm digging around in my spellbook for it. So, to supplement my action bars, I use Bartender 4 to give me the extra space I need. It also allows me to move the action bars into the corners, giving me some more real estate on my screen. (Bartender is also what let me move my reputation bar to the top of the screen.)
Prat 3.0, while it doesn't directly help me be a better warlock, helps me be a better group member because I'm not chewing my fingers off while trying to manipulate the clunky default chat interface Blizzard gave us. Well...I guess that would mean it makes me a better warlock, since I doubt I'd be very good without fingers. Prat has a multitude of useful features, including the ability to use a wheel mouse to peruse chat, as well as showing the level and class of anybody who says anything, so long as you're familiar with them. And perhaps my favorite feature is that when I'm talking in one of the /# channels, Prat remembers that I'm doing so, and next time I hit the 'enter' key, I don't need to re-type my channel number.
Similar to Prat in the way it helps me avoid a stubby-handed fate, cartographer is a much needed escape from the positively evil default map interface. I've hated that interface since day 1. You can't move while it's up, unless you were already moving when you brought it up. And that hardly matters anyway because you can't turn, or see where you're going. With Cartographer, I can resize the map, move around while it's up, and it even gives me handy tidbits of information about the zones I'm hovering over.
Then of course there are the essentials. Recount, in the lower right, is my damage meter of choice. Below it is actually another recount window. That one displays my personal DPS in real-time. And of course, if I click on the little purple bar with my name on it, I can see a pie-chart style breakdown of my damage. All of them helpful tools to maximize my usefulness to a group. In the upper right, between my minimap and my target, is Omen, my threat meter. And Sir Not-Appearing-In-Any-Screenshots is my handy-dandy raid-leading helpamajig: Deadly Boss Mods. Everyone, regardless of class or role, should probably have these three addons (or an equivalent) if they plan to do any raiding.
Those aren't all the addons I have installed, but the rest are either dubiously helpful, silly, or are going to be uninstalled whenever I stop being lazy. If I missed anything that you're terribly interested to hear about, or you have an addon you'd like to recommend I try, please feel free to comment!
A CLOSING NOTE ON NECROSIS: It sucks.