Action bars, your bags, those sorts of things are presented in a fashion that is certainly flavorful but lacks in function. At a basic level, the most important thing is getting a better handle of the things the normal UI already gives you. There are a few things you just plain need to have to make your game experience smooth, and many sources will provide you with too much, or too little for someone just beginning to alter their UI. The best Addons for WoW Shadowlands are Bartender, Bagon, Details! Damage Meter, and WeakAuras if you like that kind of stuff. Still, many users can find it hard to select the addons that will help them most. Thankfully, they encourage a robust add-on development scene that allows you to customize as little or as much as you want.
Blizzard aims for a lightweight experience, but one that cuts out a lot of valuable information and can make your life a lot harder. There’s a lot to like about WoW, but for many players, the basic functions of the UI aren’t among them.