I'm going to be blunt; the default UI just... isn't good.
Right out of the gate, the chat window is obviously way too small. But the moment you set foot in an Alliance Raid, it really just all falls apart.
Now obviously, there are a lot of robust options for customizing the UI. But as a new player, you shouldn't feel like you NEED to customize the UI. That should be something you only have to bother with as an advanced player, or if you're just REALLY particular about things. It shouldn't feel fundamentally necessary just to play the game.
Because it DOES affect the game. You have crazy elements stacking and overlapping, obstructing your view of the actual GAME, making it difficult to even see mechanics targeting the ground, let along tracking and avoiding them.
Also, while it may seem extremely minor to some, not all of the UI elements properly "line up", even when using the Grid-Snap feature. For example, if you convert your Job gauges to "simple" designs, they LOOK like your HP and MP bars. But they're a different resolution, and even when you use the grid-snap feature to try and align them, they'll always be a few pixels off.
I say this, because I feel it would be a worthwhile and relatively simple effort to simply commit to improving the DEFAULT UI setup, and perhaps even have a couple of "Presets" to choose from.
For example, you can have your "Standard" option, which is mostly how it is now but without overlaps in an Alliance Raid, or a "Minimalist" option, where everything that isn't directly related to you or your target, and probably the minimap, are hidden. Maybe even a new "Modern" option where your bars are placed to the left of your character near the center of the screen, and your target's info to the right (I've noticed this "look" has become super common).
Like I said, no need to reinvent the wheel or anything, but I think it would just be a worthwhile endeavor to improve the Default options.
Maybe even incorporate the "Class gauges" WITH your HP and MP bars, so you have a singular UI element that looks good, instead of a bunch of weird-looking elements that don't really "fit" anywhere or next to anything.
Oh, and remove the last two digits from our MP meter. Seriously, there's no reason to have a whopping 10k MP, when all of your spells cost multiples of 100. At level 1, having 10,000 MP seems crazy. Makes far more sense to just have 100 MP, and your spells reduced accordingly. Mechanically, nothing changes whatsoever, but it would make it easier to read I think.