I recommend the following GUI improvements:

• We need a way of telling what units are in a city from the Strategic Map.  I could have sworn Elemental had this showing little unit roundels on the city name and it worked fine.  Is there a reason why this was taken out?
• When viewing the Unit Details of a hero, it would be nice if we could move between the different heroes in the stack instead of closing the window and having to select a different hero.  Perhaps a simple left-right arrow similar to that in the Trade window could be used to move between heroes?
• The city list should show individual city production values in addition to gildar and research values.  I wanted to determine which of my cities had the highest production and was surprised that this information was not accessible from the City List.
  
• It would be nice to have some indication of a quest's difficulty.  For example once I've unlocked Epic Quests, I can't always tell whether a particular quest is going to be Epic, Deadly, or Strong.  The Quest giving building only gives an indication if I've got all the types memorized.
• Double clicking on a city IMO should bring up the City Details instead of bringing up the City Build menu.
• This might get messy, but I'd like an option to choose what kind of armor and weapons to upgrade a regular unit with.  For example, I tried to upgrade a unit with Leather Armor after I had Plate Armor and it listed the cost as 17,000 gildars.  I can only assume that is the cost for Plate Armor, it would be nice to have the option to upgrade to Chainmail instead at a lower cost.