Our current interface simply has too much there that get in the way of usability and simplicity.
For example, does the faction progress graph really need to be on the overworld interface? Wouldn't it have made more sense to put this in the "Kingdom Report" section? Also, a unit's actions should be front and center on the screen, not buried under a tab.
The whole interface needs more text labels, really. The most important buttons for a city is obviously for building improvements and units. You need to make the most important buttons the biggest, because your users will be using them the most.
I love how the mock-up interface had the picture of the selected unit displayed so vividly and clear. There is no question here, who you have selected. Half the time when I'm playing now, I often do not even know who or what the heck I'm targetting without squinting at the bottom.
The bottom line is that it seems to me that an artist designed the original UI and a programmer designed the current UI. I'm probably not far off from saying the same about the whole game in general. There is no "magic" or diversity in the color scheme. It all feels so plain and ordinary. Graphics are not too important to me in video games, but I know we could have done a lot better here. I mean, just look at what we're getting with HoMM 6:

I'm not asking for the game to look as detailed as this, a lot of the modding creates limitations with what you can do but... come on! You can't tell me that in 2010 Elemental couldn't have looked a little better.