I'm a big fan of FFXI's current UI. Nearly everything is out of the way unless I need it, which keeps the focus on where I want it to be: the game world.
I am hoping for a refinement with lots of available customization for those who want it. If people want to add action bars or whatever, give them the opportunity, but keep the functionality and unobtrusiveness of the current UI, please.
That being said, there are some changes that I hope for:
Seperating out chat and battle text is a must, and something I have wanted in FFXI forever.
Make mice usable but NOT mandatory. I find I really like having both hands on the keyboard. I dislike using a mouse now, and I don't want to have to re-adapt for FFXIV.
Easier to edit macros are also needed, with a wider variety of things we can do. And give us "gear sets" to make things faster.. having one line on a macro say "Enfeebling gear" that changes in all enfeebling gear would be nice (assuming we have gear swaps in FFXIV)
Allow us to move where various things show up. Like, if I wanted my macro bar to pop open on the left side instead of the top.
DON'T have hidden menus that pop up and cover important things like spell effects! If I open my macro bar, I still need to see if I have shadows up!
In the end, don't expect too much of a difference. SE will need to make a UI that works with the PS3. A lot of the reasons for XI's UI "clunkiness" was to make it work on a standard (non-HD) television... and you can use a PS3 on a standard TV. Because of that, there is a good chance that the XIV UI will also be clunky... but hopefully they will make it so that we can customize based on our setup and our needs this time.