A big "UI quality pass" is really needed over everything, and I say that knowing it was much worse before. there are a ton of minor things (like the confirm popups) or other organizational issues that are just so bad - vendor menus being the most egregious in general. Still a far cry for an actual good user experience.

On the top of my head, the ones that bothers me the most are:

1) The Party Frame. So many useless elements... I don't think I need to see a Light Party/Full Party header, nor I need to see [1],[2],[3], etc party member order labels. I don't think I need to see half of other people's buffs cluttering that list... Making all of those optional toggles would go a long way.

2) Target frame. The 'long thin line' of HP is the bane of some layouts because it's hard to fit with compact setups. I'd love the option to have it similar to how the Focus Target frame is.

3) The job gauges are lacking for some jobs. Everything that you'd need to track on the buff bar should be tracked on the job gauges as well, like they do with Huton. If people feel that it's too much, they should separate the gauges in different, toggleable, elements.

4) Retainer management. I know there must be some tech limitations to how the retainers work, but I wish it was just a single interface with tabs where you could manage them all.

5) The "You can't do [x] while [y] is active".