Unless you custom name your cities, after you have more than 4-5 you can start losing track of which city is focusing on what (presuming we get to a point where cities actually have to specialize).  It would be nice if, when a city leveled up three things would happen:

1. Jump to the city so you have some idea which one just leveled.  Show just the city for a second or two before displaying the level up window.
2. Have a different image for each city level in the popup window, just to add some flavor.  Images ranging from a small villiage to a huge capital.
3. Show the city stats in the popup so you can make an informed decision about what type of new building would be the most helpful for that city.  If you have forgotten if you were doing spell research or tech research in the city, then you don't know what building to select.  You could alternately add a "city info" button on the level up window that would open the city info window over the level up window.  What would be really cool is if you could see the city stats, and then highlight the stats that would change by building the selected building, like changing hero equipment does.  There is just not enough information on the screen to help you make an informed decision.