Really nice job on this. I have a few suggestions:
1. Can you allow access into the diplomatic overview from the trade screen? One of my biggest gripes with Civ5 is when an AI pops up during their turn and wants to be friends or declare war on someone, and I can't remember what the diplomatic situation is. This is especially so when coming back to a save game from over a week ago. Sadly I can't play everyday and keep the specific game diplomacy fresh in my head.
You could do this in Civ4 by using the function keys. Actually you can bring up any info screen in Civ4, and while this request is centered around the diplomatic overview, having access to all the function screens would be nice.
2. You're already introduced overlays by optimizing the city hovers to actually show the boundaries of each city, and what tiles are being worked (you saved me eyes from certain doom). The BUG mod for Civ4 had a
dot-mapping tool which was an easy visual way to plan for city placement. In Civ4 terms it was basically a big-fat cross (BFC) that would show the outlines of the first border expansion and show you what a city would wind up with for workable tiles. You could stamp as many of these as you wanted on the map and they would remain until removed (saved with the game).
In Civ5 terms, having an outline or overlay that was 3 hexes in every direction would be really nice, since I'm constantly counting hexes to figure out the optimal location to reach the most resources. I've had to do this multiple times since I can never remember where cities were going to go between game sessions.
3. On the leader screen and your ribbon you have cues to what the overall diplomatic standing is (i.e. friendly, guarded etc.). The leader screen clearly says it, and the borders on your ribbon I believe are the colors of the stance. Your mouseover shows the diplomatic modifiers, but not the overall stance. This isn't an issue unless you're bringing up the mouseover information by hovering over a foreign city, which is my personal preference.
4. Can you add a Civ's power rating to the leader mouseover information as well (i.e. inferior, weak, strong)? Should be the same information that prompts the numerous screens from the diplomatic adviser. There's already so much useful information there, it only makes sense to me to include this.