Creating a good UI takes planning and a seemingly endless number of minor adjustments, but finding that "perfect" UI design is worth the trouble. A custom UI can definitely improve your overall game experience because it will meet your specific needs and sense of aesthetics. There are a lot of "plug and play" UI packages out there, but ultimately the only one who knows exactly what you want is you. So why not take the time to create a UI that suits your needs?
When designing a UI, I try to meet the following goals:
- Maximum game-world visibility
- Important information easily available
- Minimize the clutter/distractions
- Make it stylish
- Keep it simple
Maximum game-world visibility: Something I hated when I started raiding "back in the day" was how little of the scenery and encounters I saw thanks to the clutter of raid frames, damage meters and the default UI. The artists went to a lot of trouble to make the game world a visually interesting experience, so why not make it so you can actually see it? From a more practical standpoint, the boss fights in Burning Crusade (and PvP, for that matter) place a lot of emphasis on environmental awareness, so the more you can see, the better. When designing a UI, I try to leave not only the center of my screen open, but the sides as well to provide a more "natural" experience.
Important information easily available: Are there certain pieces of information you always find yourself looking for? Wondering how many spaces are free in your bags? How much ammo you have left? What your frames per second is? These are all great candidates to have displayed on your UI.
Where you put this information, and how you display it, is important as well. If you have to search your entire screen, or squint to see that tiny number, you're not doing yourself any favors. Aside from making sure that these items are readable and in an easily-located spot, it's helpful to have related information close together so that your eyes don't have to move all over the screen. For example, I chose to cluster my player frame, target frame, and raid frames below my character for greatest visibility. I also placed general information at the top of my screen where it wouldn't be obscured by the game world. Any situationally-required information (damage meters, raid warnings, etc.) should ideally be toggled on and off as necessary, and placed in a distinct area.
Minimize the clutter/distractions: While some information is critical, too much information just complicates things to the point of being more of a hinderance than a boon. First of all, make sure that the information you're displaying is relevant. If you don't really need it easily accessible, why clutter your screen? One prime target is the default action bars. If you're using keybindings for these abilities, you only need to see the ones with cooldowns, freeing up a great deal of space. Bar addons like Bartender can also give you the option to hide bars unless you mouse over them, improving your overall visibility and reducing distractions.
Make it stylish: The Blizzard UI is pretty well designed, but creating a slick-looking interface that suits your style makes the game more fun. Some people may want all kinds of bells and whistles, with 3-D portraits and fancy art, while others want very simple, utilitarian looks. Make your UI match your personality.
Keep it simple: There are a lot of great addons out there that can help tailor your UI, but I've found that the really complicated ones can sometimes be a hassle. If updating your mods requires completely reseting your UI, you may want to think twice before devoting a lot of time to learning a particular addon.
Once you have an idea in mind, take a little time to review exactly what you want to do. If you only want to make a few minor adjustments, that's easy to jump in to, but if you have lots of changes you'd like to make, you'll want a good idea of exactly how each part should be set up. You'll inevitably have to make changes along the way, but minimizing them from the outset makes the process less onerous.