As that screenshot, dated in 2002 shows, UI customization in MMOs has been around for a long time. I can't remember exactly when they added the capability to EQ1, but it was probably a year prior to that shot. Since then, interface customization has grown by leaps and bounds!

EQ1 only allows "skinning" - that is, all you can change is how and where things are displayed. You can't actually "program" the UI like you can in WoW and Warhammer, but you can still do a lot to the UI.

The evolution of my EQ1 UI...

I spend a lot of time designing and tweaking my UI in games that allow me to. I often say I "can't" play with the default UI on games like WoW or Warhammer. That's not exactly true - of course I could play on the default UI setup, it's just that with addons and customization you can really get a UI tailored to how you think and react, and I believe you end up a better player.

A longterm friend and I both heavily play this metagame, and we often spend hours on IM discussing UI and keybind setups. It really is a detailed, complex metagame.

The more modern MMOs, such as WoW and Warhammer really let you change the UI to suit you. Both of those games have a full-blown programming language, LUA, built into the client, and they expose much of the client API to the UI, allowing you to really make cool modifications to the game's UI.

Let's start with WoW's default, "as installed", UI:

Certainly that's not "unusable" but from my point of view it has several problems that are easily fixed. First, the unit frames, that is, the displays of the player's and the target's health and such, are up in the corners of the display, away from the center of the action, where your eyes should be focused. Also, the hotbars display and other UI elements are a little too "heavy weight", taking up a lot of screen real estate.

Here's my WoW UI as of December of 2009:

Note that the unit frames have been pared down to just what's needed, and most of the UI elements are relocated to a "focus zone." This isn't the best the UI could be; I'm always refining and tweaking my game UIs.

The two main things I first look for when seeking addons are a lightweight unitframes mod, and a good hotbar mod. Most game's default unitframes are pretty overblown. They usually have a large, animated portrait of the character, which wastes both space and CPU cycles. They're also usually too big. So I look for a good, clean unitframe mod. For hotbars, I need something flexible and that reminds me what keys are bound to what. I'm a keybinder - clicking simply takes too long. Hitting a key is much faster than moving a mouse and clicking; there's just no way around that.

After those two things are handled, I generally try to find a good quest log managing mod, a good minimap mod, and if it exists, a good "Grid" type party/raid frame mod. Other things I look for/use are totally specific to the game - such as Recount for DPS/healing tracking in WoW, or HorizontalMechanicBar for tracking class mechanics in Warhammer.

So - what about you? Do you heavily play the UI Metagame, or do you just install the game and play with what "The Man™" gave you?