I've found Power Auras to be a really useful addon over the last few weeks, and it has dramatically improved my performance since I added it to my UI.
Basically it adds a large customisable graphic whenever you get a particular buff, and you can add a sound as well. This is brilliant for dps roles which rely on procs - I use it for "Slam!" when I'm playing in my fury offspec, and for "Missile Barrage" and "Hot Streak" procs on my mage alt. Since these procs add a short buff, it's very easy to see if you have an ability proc - without the addon I had to watch the scrolling combat text to see if the proc name came up, or look at my (crowded) buff list. If my attention was diverted (eg I was looking at my rage bar to see if I had enough rage to heroic strike, which I do every three seconds) then I would simply miss the proc and take a dps loss.
With Power Auras, it's obvious straight away if you have a proc active and so you shouldn't miss out on any dps from proc events.
So - great for dpsing. But there's more.
Rotating cooldowns is now a key part of tanking, and Power Auras helps here too. You can assign Shield Wall and Last Stand to the addon (for example), so that you can see clearly when you have each buff active. This means that if you are chaining cooldowns together you don't have to watch the buff timer in your buff list, and there's no possibility of you forgetting to activate the second buff - you can see when Shield Wall has ended and know that it's time to activate Last Stand. It makes fights such as Heroic Northrend Beasts and Festergut much easier (these are particular fights where you need to chain cooldowns).
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