Let's start with some of the fun addons first! One of my favorite addons, especially with the often-heralded 2 piece Tier 10 bonus, is TrickOrTreat. Not only is it functional (it alerts you when you give/receive Tricks of the Trade), it also lets you know exactly how much threat you redirected to your target and how much of a DPS boost they received from your Tricks.
Sometimes it can be hard to visualize the raid damage boost that TotT provides, and it can feel like you're just tossing a GCD down the hole. TrickOrTreat solves that problem! I love to use it whenever I'm doing one of the old instances for the weekly raid, as my guild mates will have competitions to see who can pull off the biggest damage bonuses during my TotT duration. I'll even toss in Glyph of Tricks of the Trade just to give them a higher goal to shoot for. It promotes friendly competition, and adds some excitement to the boredom of your 40th Patchwerk kill.
Another fun and functional addon I like to use is called "I Interrupted That", which announces your interrupts only when they succeed. I used to simply have my PvE Kick macro that included a raid message indicating that my Kick was on CD, but it was terrible to use when I was spamming Kick as it was cooling down. By monitoring the combat log for the actual interrupt event, IIT replaces all the clunky Kick macros and works out of the box. I find it particularly useful on 25 man Lady Deathwhisper, where a missed Kick opportunity cascades into a ton of burst damage onto your tank.
One of the reasons that I'm not too excited about the new Rupture buff is that I had gotten used to having one less timer to watch in my DPS rotations. Adding Rupture back to the mix will mean another limited duration ability to monitor, more complex flowcharts for figuring out when to use which finisher, and draws more of my attention to my UI instead of the encounter itself. Luckily, there are a plethora of great timer mods out there to help you manage your rotation with the minimal amount of effort. I've used RoguePowerBars, NeedToKnow, Class Timers, and ForteXorcist; these 4 addons are all great for monitoring your abilities and timers.
My favorite, however, is called EventHorizon. Why? Because I'm a sucker for a cool graph.If you're new to a spec, I'd recommend trying out something with timers and bars first, as you'll want to get the hang of managing your build first and then work on fine tuning it later. Once you get a feel for your rotation, EHZ will let you maintain control over your rogue without needless numbers all over the place to confuse you.
Every boss encounter can be summed up in five words: "Don't stand in the fire." The only thing that changes from fight to fight are where the fire is located and how shiny it happens to be at the time. While boss mods of some sort have become pretty ubiquitous, there are a few specialized addons that will help you at a level above "there's some fire coming". One such addon type is the threat meter, and you really have two choices for handling this. If you like to use Skada for your local damage meter, you'll be happy to know it already has a built-in threat monitoring feature. Otherwise, you'll be downloading Omen to handle watching your raid's threat in real-time.
The key is to watch Skada or Omen and remember who's close to the tank in threat. You don't want to be giving Tricks of the Trade to the fury warrior who's at 109% of the tank's threat, because you're just asking for him to get smashed into a million pieces. You also don't want to give Tricks to the wrong tank on a fight that involves a serious risk if the off tank picks up threat, like on Saurfang or Festergut. Rogues who spam TotT rarely run into threat issues, and we always have Vanish to fall back on in the case of an emergency. The key is to use your threat meter as a tool to ensure you don't accidentally Tricks to the wrong target and get yourself cleaved, because as we all know, a dead rogue does no damage.
Quality of life
There's no rule that says that every addon you download has to help you up your DPS or do some amazing task. A few of my favorites are very simple addons that assist in my day-to-day life as a rogue, and make playing just a little bit easier. Poisoner is a great example, as it saves me some screen space and I don't have to worry about cluttering up my bars with poison macros. It's just a tiny button that lets you choose which poisons to apply at will. Another addon that I couldn't play without actually removes functionality from the game. ErrorFilter suppresses a lot of the error messages that have plagued the rogue class since day 1. We know we don't have enough energy to cast that move, that's why we're spamming it while waiting!
Addons have become as large a part of the game as consumables or even gems. You're expected to have a certain minimum level of UI tools available to even attend a raid. As we transitioned into WotLK, we saw the death of Energy Watch, and the birth of smooth energy regeneration. The complex assassination rotations of level 80 were literally painful to play without several different timer bars to let you know when to Rupture, when Deadly Poison was about to tick, and how long until you had to refresh HfB. We saw the rise and fall of PoisonSwapper, the most controversial rogue addon yet. Clearly, addons are here to stay and will only become more and more prevalent amongst the community. Try looking for areas in your own UI that are lacking, and then search for a mod to solve that problem. More automation via addons means more time for you to sit back and enjoy whatever content you're actually doing.