Awesome game. Some suggestions respectfully submitted.
1. Interface: Always show current revenue/expense (maybe at the bottom left next to your treasury amount). I need to switch to the planetary sidebar screen constantly to figure out what is going on.
2. Diplomacy: Currently, whenever a proposed deal fails, all the items are taken off the table. It is very tedious to readjust each proposed deal by clicking on all bargaining items all over again to find the correct mix.
3. Trade: Notification when tributes will expire - i.e. if you have a deal to trade a technology for 10 turns of 50 credits, it would be nice to have an idea (listed somewhere) when my revenue will decrease.
4. Tighter slider bars for military/social/research bars. Not to make comparisons, but the Civ3 people did a great job of tying bars together, so if you move 1 up, graphically the other 2 would move down (and you could lock 1 slider out of 3). Your system does the same thing without the graphic represenation. It would look nicer and more intuitive.
5. Echoing a previous suggestion: A picture or movie or something for a defeated race. At least a message!

Reward us for our hard work at defeating your painful AI opponents. You guys could even have fund designing a different one for each major race and one for the minor ones...
6. Some explanation somewhere of more game mechanics like how invasions are calculated and what if any effects different political parties have on your policies - does it matter if you want to declare war and the pacifists are in power? Etc.