I was forced into buying a new computer the other day when mine died an abrupt and tragic death. With a new computer comes hours of UI set up and I took advantage of the opportunity to tweak a few things. Here is my affliction warlock's UI as it is now.
First off you know you love my little gnome artwork. You might not think you do, but if you think about it you will come to the realization that it is indeed the best thing you have ever seen.
Sunn Viewport Art- allows the the panel at the bottom, providing a dedicated space for addons.
Sexy Map - My minimap is now movable and sexy!
Recount- double duh
Dominos- easy to use bar addon. Things like professions and consumables are on bars that are faded out until I mouse over them, a feature I enjoy.
Class Timers- every warlock needs a Dot Timer. The little icons in middle of my screen count down the time left on my dots. You can have bars, but I like the way the icons take up so little space. This addon is extremely easy to use, works well, and works great for all classes.
Arkinventory- I love this addon. I allows you to set custom categories for organizing your bags. It can be a pain to set up, but it is well worth it.
GupPet- my newest addon, it puts out a companion and summons a random mount. It also includes a nice feature for collectors that will list every mount or pet that you don't currently have.
Deadly Boss Mods- If you don't have it, get it, right now, I'll wait. Got it? Good.
I am no UI wizard, but I think I've achieved a very functional UI with a decent look with relatively few addons. Perhaps I'll post the tree's UI soon.