For me the UI is the biggest let down in this game but in all honesty it won't stop me playing this game, however some people who are not used to playing games with bad UI might think this game is horrid.
Here are a list of things i can think up off the top of my head (Just so you know my resolution is at 1920x1200)
1. When i click on a building to build something the icons are really small and you cannot distinguish between most of them meaning you have to scroll over them to read the tooltip. Maybe they were designed to run at a lower resolution?
2. Text isn't the most legible, it is passable but i still think the font or something could be better. Once again it could be a resolution issue.
3. Creating an army for the trade mission was probably the most confusing thing i had to do so far. When i go out to the main map maybe when you select your town a radial dial comes up with an option to create an army, just to keep it in line with how you have done building things. The box down in the left hand corner was a bit confusing at first, i had to read the help section. Once you understand how to do it, it no longer is really an issue but i saw a few people in chat asking the same thing yesterday.
4. Quest dialogue could be made crisper, add some spacing between the dialogue and the options.
If i think of anything else i will make another post.