It's great o see a thread like this on EJ. Usability and interface design are two interests of mine both in-game and in my studies. I'd like to focus the discussion a bit more on the general usability aspects of Rogue GUI design rather then going in to the specifics of which addons to use exactly.
When talking about a system like World of Warcraft it often has to do with how we understand the program and how it works. The more we understand of it the more likely are we to start using addons or modifying how we work with a system. Or rather how we think we understand it and how we think it works, this is called mental models. In WoW/EJ terms this has in some to do with the theory crafting going on here. The more we read up on different theories and in-game tests of theories the more we evolve our mental model of the game. The basic standard UI of WoW is pretty straight forward and very general. A new player doesn't need much information from the game to get going but as we understand more and more of the game we also need more and more information from it to better our own performance and at the same time we no longer need some of the information that used to be essential to us when starting the game 3 years ago.
One such example is the error messages "ability not ready yet", "not enough energy", "not in range". After having played the game for so long we know how and when these things happen without actually having to read the error messages. There are two ways around this, either write a macro for every single spell that turns off the error ui, or download an addon that filters out the red error messages. One such example is ErrorMonster.
The easier, and quicker we get relevant information however, the more likely are we to better ourselves and improve our own performance. So this is where UI-design comes in as well as what sort of information we choose to display with addons. I'm going to focus this post on where and how we place the objects of our UI to get the shortest response time.
Len did go through a lot of the basic ideas about this on the first page of this thread. Such as placing important objects in the middle of the field and placing similar items together. I'd like to go into a bit more on placement of objects and the background behind why we place elements in the center and where in the center of the center we place them.
Eye-Tracking
Traditionally most eye tracking studies have been done on people reading online news or other forms of textbased interfaces and usually these studies show us that the most important zone of the screen, "priority zone", is the upper left corner because that's usually where the eye goes when first seeing something. A major part of the standard UI in WoW is focused around the upper left corner.
Recently some studies have been done on eye-tracking in gaming and 3D gaming in particular. One such study was made at the Department of Cognitive Science, Lund University. They show that a person playing a 3D game, for example a third person shooter like Counter Strike often behaves similar to how he would behave in a real life situation. His eyes are focused on where he believes the threat to be biggest. For example when rounding a corner the eye tends to focus on the inner side of the corner, very similar to where the eye is focused when driving a car. However when the test person was moving through a "corridor-like" environment his eye sight was focused on the floor and about one third up on the walls and floor coming in front of the character. In other words on the lower center part of the screen and up to the center.
Visual Fields
Center
The center field is the the most important one, not only because of the eye track studies, but also because traditionally the center of a picture or a workspace is where the main task is done. This is where the essentials of the game is going on and this is where our eye sight is going to be focused most of the time. So we should place objects here that we need to look at continuously during a fight without loosing focus on what is going on around us. In other words, we want as little eye movement as possible to not loose focus. Placing objects in the center of the screen also means that we should be able to see them with our peripheral sight without loosing focus on our target. As the OP said in some earlier post this is a balancing act between not getting the close enough to our attention field and cluttering up the screen so we don't see what is going on.
So only place information here that is essential for solving the task but without blocking what is going on in the game. In my opinion you can scale down objects and still easily see them in your peripheral so there's no point in oversizing addons or meters unless it fills a purpose. Based on eye tracking it is also a good idea to place important objects centered below the characters feet rather then above his head because your eyes are more likely to be focused on the lower center part of the screen than the upper.
One tip is to place boss alerts with a long duration in the peripheral right and another timer with the short duration centered under the character to definitely awake your attention.
Peripheral
Objects placed to the left and the right of the screen are to function as support elements to the center. These objects are placed in our peripheral sight and we should rarely have to focus on these elements in mid fight. It's alot like not having to look at the pedals or the gear stick when driving a car but you still use them. You know the information is there but you really don't need to look at it to solve the task. Generally things we expect, such as party/raid frames are recommended to be placed to the left and new information such as long-duration-buffs, -debuffs and -boss alerts are placed on the right side of the screen.