For all of the following pictures, if you click them they should enlarge, just like... never mind.
Here is a screenshot of me in a 10 man Trial run with my guild. All I want you to focus on here is the clusterfuck of addons that I have. But trust me, apprentice, clusterfucks are good. I'll go over all the addons you can see on my screen and why I think they're important.
GRID
Grid is an addon that takes your party and raid frames and condenses them into little boxes, as seen here. The little boxes are colored by class (pink for paladins, thanks Blizzard). When someone loses health, the box becomes emptier (look at the orange box and you can see that our druid has lost some health). People out of range will be grayed out, and dead people will have a gray box with DEAD in place of their name. Of course, this mod is highly customizable; you can change the shape, spacing, coloring, and shading of the boxes, among other things. I like it because it saves some of my screen real estate. Yeah, that's pretty much the only reason.
PALLY POWER
Next up is Pally Power. As a paladin, this could be the single most useful mod for paladins in a raid environment. What it does is condense your blessings, seals, and auras into a little strip. What's unique about this mod is that it allows an individual, with either lead or assist, to assign who gives what blessing to whom. The assignments will show up in the main window, such as above, and all you need do is click each class to give them their buff. Or, if you're REALLY lazy, you can click the little Badge of Justice-esque icon below your seals many times and it'll automatically apply the buffs in a sequential order. Neat, huh?
CHATTER
All Chatter does is retool your chat interface, colors names by class, includes their level, timestamps, and other stuff. One cool feature is that you can click a button and open a chatlog that'll allow you to copy stuff from your chat. Not entirely necessary, but nice.
DEADLY BOSS MODS
Deadly Boss Mods is a must-have for raiding. Wrathy, from Avenging Wrathy, has pointed out an alternative to DBM that is called BigWigs; I've never used it, but if you don't like DBM for some insane reason, try that out. What these addons do is alert you to special moves that bosses have, when they're happening, how long they have on cooldown, even what raid members are affected by what debuff. Excellent for those with situational awareness issues (i.e. myself), as it plays noises and flashes your screen when shit goes down.
PITBULL
Pitbull just redresses your raid frames from the standard pilgrim smock of Blizzard's UI to a sexy, slimmed down string bikini. Highly customizable, highly confusing (the two kind of go hand in hand). Don't let that detract you, though; I much prefer these frames to XPerl. I have my focus, self, target, and target of target frames in the middle of my screen because those tend to be the most important.
POWER AURAS CLASSIC
Oh, how I could sing a lovely song on the wonders of Power Auras. This mod is so customizable, it puts Build-a-Bear to shame. Simply put, this mod makes visual and/or audio signals that are triggered when an event that you specify occurs. Huh? For example, when my Art of War procs, I have it show up as a yellow, glowing circle, as seen in the picture above. I also have a timer that indicates how much time I have left on the proc. For Prot, I use Power Auras to signal when my Divine Plea drops and my Holy Shield needs to be refreshed. The uses for this mod are endless. Not a necessity, but it'll make your virtual life much much better.
DOMINOS
Dominos replaces the action bars from the Blizzard UI with highly moveable and customizable bars. You can move them any which way, on whatever scale you desire. Personally, I don't like clicking my moves, so my bars are really small to discourage me from clicking.
MSBT
Mik's Scrolling Battle Text (or MSBT) replaces the default floating combat text in the Blizzard UI. With MSBT, you can control the size of the text, the positioning, all that fun stuff. Useful for clearing the center of your screen of Blizzard's floating numbers so you can see that boss's cute ass better.
QUARTZ
So I failed and didn't get a screenshot of the ACTUAL use of this mod in action, but use your imagination. Quartz replaces your cast bars with better looking ones (they're smooth and shiny), but more importantly, you can move and scale them. Also, as seen in the picture, the buffs and debuffs that YOU applied to the target will be shown above the target's cast bar.
OMEN
Your tanks will thank you for installing this addon. It shows your current threat on your target, as well as how much threat you need to produce in order to pull off the tank. Gives you a good goal to shoot for. Seriously though, get this.
SEXYMAP
Completely and totally unnecessary, just makes your map look sweeter. There are different styles to choose from, and you can customize which icons you see on your minimap and which you don't.
RECOUNT
I saved the best for last. You aren't a TRUE ret paladin; nay, a true DPS, if you don't have this mod running. It keeps track of other things (who tops dispels on IC? this guy!), but they're not as important. The two graphs I have below my Recount window keep track of overall raid DPS and my latency (ping).
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So there you go, take from my UI what you like and leave what you don't. Pally Power is a requirement in my guild, as well as most raiding guilds you'll join. When your UI turns you on, then you know you've done a good job.
