I posted about this before but got no response. I feel this is an issue that needs to be fixed soon.
Diplomacy is FAR TOO POWERFUL.
You can win extremely easily, even on suicidal difficulty, by going through this arduous process each turn: Speak to all of the other civilizations, purchase any new technologies they developed on that turn, and sell their technologies to all the other civilizations. Finish your turn, and repeat.
On top of this, you can research your own diplomacy technologies and withhold them from the other civilizations, which will increase the cash you receive from their technologies. Eventually you will have so much cash that you can purchase their mining starbases, and control all the galactic resources. At this point, winning is simply a matter of time.
The best fix would be for the AI to consider how many other races have a technology in their value estimate. If you are selling them a technology which only you own, they should be willing to give up a lot of credits for it. If you are selling them a technology which all the other races own, the technology should be nearly worthless.
This should go for technologies that you purchase from the civilizations too; if a civilization is the sole owner of a technology they should charge a lot for it, and if all the civilizations have it, it should be fairly cheap.
- Fragsworth