In general I have been enjoying my time with BG3. One of the easiest things to improve for me would be the UI.

My main problem is the general action bar, which combines items, spells and actions, but doesn't organize them. It's almost a mini game upon itself to keep this bar clean and useful for all your characters.

My main problems:
1 Different types of abilities combined
2 Abilities not linked to their resource
3 Different buttons for melee and ranged version of the same ability
4 Different buttons for different levels of the same spell
5 I have to rearrange this bar manually after leveling up

The way I think about using these abilities influence how I would expect them to behave.

I think about which resource I want to use. Which leads me to prefer a separate UI element per resource type, which shows my resource (including how much I have left) and grouped with that, all the abilities I can do with this resource. For spells I would like to see all the spells groups with their lowest level (each level also shows how many spells slot I have left) (Tackling problem 1, 2 and 5)

To cast a spell I first think about which spell I want to cast, before thinking which level spell slot I want to invest. So to me it makes sense to first select the spell I want to cast, which would select the spell on the lowest possible level. Then give the player the opportunity to select a higher level. Personally I would do this my making the spell slots resources interactable. This visually links my action to the prerequisites (do I have a spell slot for this level) and consequence (I will use a spell slot of this level).

For different buttons for melee and ranged, when I first encountered this, I assumed that I would simply select my weapon, melee or ranged and then select my ability. This ability would then be performed with my current selected weapon.

Oh, yes, small pet peeve, the general actions (disengage/hide/etc) aren't always in the same order.