Tip #27: See What You Need
So much attention has been spent to good UIs. There's a reason. You don't want too much information. You need some of it summarized and some of it in your face. You can't have too much or you'll tune it out -- and maybe tune out too much. And if you don't tune it out, it will delay your other thought processes. You need screen real estate. You need good addons. You need to try out the features of the UI you make so that it works for you.
Do you need numbers spiraling out at you? Do you need to know who healed your pet? Do you need numbers counting down the milliseconds before the graphical bar shows it is complete? Do you need to see your pet casts? Do you need a timer bar for your Blessing of Kings buff? Think about what you need to remove from your UI as much as what you need to add to your UI.
After you boiled down what you need and don't need, make sure to have space to see what is going on around your character. Clutter is not your friend. If you're using Power Auras, don't make them so they obscure your general vicinity. (Power Auras is a great visual space-saver, but only use it for what is important!) Cast bars can be thinned. Keep what you need to look at front and center. Keep things that you scan quickly to the left and right. Humans, like other animals, grew up in the landscape world and have eyes horizontally aligned. We'll see changes and horizontal movement a lot quicker than a vertical change.
There is a thread over at Elitist Jerks with over a 100 hunter UI screenshots. Check it out for ideas.
I recommend Power Auras for the important stuff; Need to Know for the important procs and target debuffs; and Mike's Combat Scrolling Text to keep the information flow reduced and flowing out of the way. And of course everyone recommends something like Deadly Boss Mods and something for combat stats. There's also help for auto-shot visualization, and viper alarms.