This is the start of a series of UI design posts. I figured before I start talking about how to design your UI and make it functional for you, I should discuss my own.
First, we have the DPS UI. This is Lyllea's.
The most important thing (other than not standing in BAD) is MFClip, which lives just above my bars. This keeps track of my dots for me, in a nice visual package. Next to it is Decursive, which is fairly central so I notice and remember to dispel and cleanse people. Omen lives in that blank space to the left of my bars, just below Vuhdo. Vuhdo itself is off to the side so I don't forget I'm dpsing and start healing. I have what I consider to be a nice clear field of view. My minimap's faded out so it doesn't distract me, and skada would be faded out if I weren't dead.
Then we have the healing UI. Same basic principles, emphasize the important stuff, that sort of thing, just much more going on.
Now, my UIs could both be improved. Just looking at it, I need to figure out a better place to stick the casting bars in disc, trim down the amount of info my HUD (heads up display, the stuff right around my character) is showing me, trim down what battle text I'm seeing, convince Skada not to show in combat, and maybe look into a neater buff solution. Also, it'd be really nice if I didn't have to display both the blizzard buff frames and my addon ones just to click off a buff.
But any UI is a work in progress, and I am quite happy with what I've come up with thus far. And both screencaps are just decent shots from my screencaps folder. First one is a rather epic heroic bloodqueen kill, and the second's from a fun run of Sunwell.