I've been pondering this for a week now and I just can't narrow it down to a single item, so I guess pick one of these.
The Trade Window
1) On the Trading Page: Add a means to limit the goods displayed. For example, I no longer have any need to see anything before early middle ages.
2) On the create offer page:
2.1) Either Lose the auto-fill zero or select it so it's easier to over-type.
2.2) Stop auto-correcting my entries and move the edit check to the [Create Offer] button. Worse of a pain because of (2.1) but presume I'm in the middle of creating multiple trades.
2.3) Allow quantities to be entered without a good. When I do trading I prefer to set the quantity and then alter the offer and need.
2.4) Could use a means to limit the displayed goods as well.
3) On the "Your Current Offers" page, allow sorting by "Offer" and "Need".
General Numeric Formatting
1) Please add commas! 1000000 is a lot easier to read when displayed as 1,000,000
Neighborhood / Guild / Friends list
1) Allow me to alter the number of players displayed so I can take advantage of my full screen width.
2) Add an indicator to the portrait for number of days since last played. So I can see who won't m/p me back.
3) Clicking on my portrait should take me home.
4) When visiting multiple players, I really don't need to drop by my city first.
A City Inventory Display
To help identify roads / buildings that need updating
Promenade - Qty:72 Happy: 35 Age: Progressive Era
Sett Paved Road - Qty: 5 Happy: 30 Age: Industrial Era
Tree - Qty: 1 Happy: 18 Age Stone Age
Victorian House - Qty: 36 Pop: 474 Coins: 2,420 in 1d
General City Display
1) Sometimes it is very difficult to locate what building is ready to collect. Could you add an indicator that would work like the "Idle Indicator" to help find them.
2) When in build mode turn on the layout grid so I don't actually have to start moving a building to see it.
Quests
1) On the "Pay" quests, after a player accepts the quest, display the quest window so a player can just click the "Pay" button to finish the quest instead of having to click the quest icon. Basically, make it work more like quests where a player needs to acquire an already owned sector.