Personally I've only used add-ons that make my UI even more minimalist then the original WoW, whilst still adding a lot of functionality.
X-Perl Unit frames (In my mind the best Unit Frames add-on ever, sleek, simple, functional, highly customizable)
Bartender (Action Bars, all movable, individually scalable, with a small variety of textures, also adds a new action bar for Druids methinks when they change form)
Wardrobe (for quick equipment changes depending on scenario)
BloodyFont (changes the basic WoW damage numbers to a particular Font you wish, simply insert your own)
Omen (great threat meter, off unless I'm on bosses or Raids)
Chatmod (adds new functionality to the chat box, such as copying websites etc)
Simple Minimap (movable minimap, shape, and can remove all the extra stuff you don't need, zoom, time counter, location name, etc)
Detachminimapbuttons (as the add-on already suggests, can be problematic at first, some are moved off screen the first time, so be sure to set this add-on first before moving the map location)

Apart from that

other useful ones would be:

Quest helper (altho I've lvld too much in my gaming life so i tend to know most of wow pre-tbc like the back of my hand (Horde), tbc this can come in very handy)
Atlas/Atlaslootenhanced (for all ur gear drooling)
Proximo (if you decide to do arena, personally I've had problems with this add-on, but those were due to my crappy pc)


I have got my screen down to chat, minimap and 4 actionbars at the bottom, most of my spells are keybound, so i don;t have loads of abilities on the action bars, mainly buffs. then the only other item on my screen is the party list, usually very minimalist, top right corner.

The more screen is left for your game the more enjoyable it becomes Too many times have I seen UIs that are soo cluttered that one could say all the graphics, scenery etc are lost on some players.. would be as if the game were text based :S