You don't need Lua or XML based addons to make a good UI. Any good addon will do. This is what I recommend when building a UI from scratch:
Just sit down and think what the shortcomings of the default UI are. Take care of the basic problems, like centering, positioning, removal of unnecessary artwork, among other things. This is where your role and class come into play. Figure out what
you want and need out of your UI.
Once this is done you can start adding features you wish the default UI would have: timers, cooldown trackers, threat meters, boss mods, etc. Try to get basic positioning out of the way at this point. Make sure important things are closer to the center or where your eyes are focused most of the time. Unnecessary things can and should be moved to the sides or removed altogether. Size matters, make sure things aren't too big or too small, depending on their importance.
Try to see if you can use the same addon for more than one purpose. For instance, use your unit frame addon for buffs, debuffs and timers instead of downloading a separate addon for each one. Or use an addon that tracks both threat and damage. This helps reduce addon clutter and memory/CPU usage and in the long term will make it easier to maintain and update your UI.
Once you feel you have every feature you need and have found good positions for every element of your UI, you can start thinking about skins, panels and artwork. Try to find a unifying bar texture and font for all or most of your UI. Don't have more than 3 fonts in your UI. Decide if you want a general color theme or class/reaction colors. Decide if you want/need panels and artwork. Make sure these follow a certain theme. Avoid unnecessary artwork clutter. Avoid artwork that makes it harder for you to get information out of your UI.
Once you think you are done, start over. By which I mean, go back to the beginning and make sure all your bases are covered. Are the shortcomings of the default UI solved? Are important elements for my class and spec clearly visible and accessible? Does my UI look clean? Does it have a unifying theme? Make small changes and adjustments, join a few pugs, make sure everything is working properly, make sure you have everything you need. Think of all possible scenarios (vehicles, boss-specific mechanics, pvp) and what you might need in them, and make sure your UI provides them.
Then show off your UI. Ask for criticism. Do not be afraid of criticism. If someone tells you to position your unitframes that way, and you say no, be prepared to justify why you want them the way they are. Each UI is unique, each person has a unique playstyle, but you should always strive to improve yourself and find new, better ways to do things.
Hope this helps
