To me diplomacy and trading seems like a total scam towards AI 
Somehow AI doesn't seem to be very good at mining resources, or maybe AI just looses units much more often than I and consequently is often short of resources.
If AI has a lot of crystal/iron you will get no value for it in the trading window, but if AI is short you can easily trade your crystal/iron for all the tech/gold computer has to offer.
I do have solutions for this problem and the one above where you can scam AI for the war you would have declared anyway:
1) AI should trade each other more aggressively (I am not sure if they do at all now) so there are less benefits for the human player to reap
2) If AI isn't on the good terms with you, it should perceive your offer at a lesser value just because you aren't likable
3) AI should attempt to "scam" the player out of gold by offering to declare war on someone just like a human player can, and then if human refuses to pay actually declare war on the human once in a while.
In general, diplomacy and your behavior should have more long lasting consequences as in Civ series. I know it must be tough to be constantly compared against the Civ and I feel bad for devs who came up with a pretty good game, but if you are going into the 4X field you gotta match the standards set before (and hopefully exceed them). This is what makes a difference whether I will play the game for 30 or 300 hours.
Anyway. One particular suggestion regarding diplomacy is that AI's priorities should be set not on just winning, but also on preventing you from winning if you are close. As in the underdogs should gang up on the top dog. I remember in Civ on harder difficulties god forbid you somehow manage to get ahead too early, everyone and their neighbor will be knocking on your door. Here the stronger you get the more cowardly the AI becomes.
