The main issue with the UI is the way the UI is built up, in my opinion.
Currently every element is simply laid on a 16:9 canvas, which is then stretched so it fills the game's resolution.
On non-16:9 this means the UI goes out of it's proportions. On any resolution the data on the UI is blurred/scaled somehow.
Look at this screenshot of TM on 3 screens:
http://www.wsgf.org/f/u/imagecache/node ... me_3x1.jpg - The UI is always scaled 3x in it's width (if the screens are all 16:9) due to the way the UI works.
Using the current (ancient) UI system it is impossible to make e.g. a 100x100 pixels image appear on the screen, as it's always blurred/scaled somehow, different for each screen as well.
Because of this it's impossible to make nice-looking stuff as well. Only primitive things can be shown 'nicely' (e.g. a rectangle doesn't lose much quality if you scale it).
The current system is using a complete relative metrics system. A severe improvement would be to support both relative and absolute metrics (e.g. a position of {0.5, -200; 0.0, 100} means a position at the center of the screen, then 200 pixels to the left, 100 pixels from the top). This metrics system is used in quite some other engines as well.
In my opinion the current system simply limits the UI designers at Nadeo to make better looking UI's in first place.