It's more fair to say that a lot of the UI for WoW is written in Lua, and most of the API available to that UI is also available to addons that end-users write

It's more fair to say that a lot of the UI for WoW is written in Lua, and most of the API available to that UI is also available to addons that end-users write for the game. You can take a look at the UI code, since it's dumpable from the game client: http://wowprogramming.com/utils/xmlbrowser http://wowprogramming.com/utils/xmlbrowser