The ability to move individual parts of the UI is awesome (and something that is lacking by default in various other games) however - I would mind the ability to change some other things:

- change horizontal hotbars to vertical or vice versa.
- change the stack size of a hotbar. Instead of 1x12 it can be altered to 2x6 or 3x4 (or 6x2/4x3 if vertical).
- more chat boxes
- the ability to change the font size to 8 or 10 point.
- the ability to change the layout of the party/alliance frames into something that isn't vertical.

Hmm, I've used the word "ability" far too many times, but I'm too tired to care.