I have noted some unfavorable quirks with diplomacy. Here I will share a few;
1) A civilization will go from friendly (or at least non-hostile) to war without any warning whatsoever, even if I have a diplomat assigned. What the hell? My advisors and diplomat should be able to give me some sort of warnings. Instead I get the useless "Beware the "X", they really hate us." AFTER the war has started...
2) A civilization seems to only consider that "we are trading" not the quantity or value of the trade.
3) A civilization will attack me, get their ass handed to them, then a few turns later ask for peace. However I am not given the same opportunity as I am constantly getting "too soon after declaring war". Even when they just contacted me TWICE.
4) A war will get 'permanent' status and I am unable to ask for peace, yet that civilization will inevitably request peace from me. WTF?
5) I will request a civ to declare war, which they will do, and then only a few turns later I will see that they are at peace. As with #3 above - WTF?