I think the issue that most of us have with diplomacy in this game is that exists primarily to screw over the human player.
I understand that. The human has a sizable advantage over the AI on even the highest difficulty settings, as it is just not currently possible to program an AI which can adapt to changing situations as efficiently as a human can. The diplomacy evens that out a bit by having a decidedly anti-human tilt, effectively allowing the AI civs to gang up without forming an outright team in the game. Totally understandable as a game balancing ploy.
The problem is that it takes away from my enjoyment of the game. Here, let's run through a typical gameplay scenario:
-Set up the game, pick a primarily peaceful civ with the intention of playing a nice, relaxing culture or science game.
-Turn 15: meet my first neighbor. It's Japan.
-Turn 27: meet my second neighbor. It's the Aztecs.
-Turn 53: get DoW'd by both AIs. At this point it's obvious I'm not going to be able to play my intended game, leaving me with two options: quit & restart, or shift into defensemonger mode & play the rest of the game either fighting never-ending wars on my borders, or being universally hated by every other leader in the game for defeating Ida & Monty and "stealing" their lands.
Or, alternately:
-Start game as trade-based civ. Luck into peaceful neighbors, reign in my expansion so as not to upset anyone, and play all the way through the mid-game holding friendly relations with every other leader in the game. Sign DoFs, RAs, trade/sell luxuries, and get the game I was looking for. Until...
-Turn 282: Two turns after re-upping my long-standing DoF with the Netherlands, for no apparent reason whatsoever Willy decides to denounce Arabia and Ethiopia (who had been mutual friends).
-Turn 283: Arabia and Ethiopia issue denunciations of the Netherlands & warn me about getting too close to them. I apologize.
-Turn 284: Arabia and Ethiopia denounce me.
-Turn 285: The Netherlands denounces me.
Pick your flavor--the end result is that I end up playing the same freaking game every single time--build 2-4 cities, get DoW'd by 2-3 AIs, crush the invading armies, accept peace with 2, take the third down to one city, get denounced by everyone else for being a warmonger/expanding too rapidly/encroaching their borders/owning wonders (via conquest) which they coveted. If I manage to play through an entire game without being forced to conquer at least a quarter of the other civs, it's either an accident or I've dropped down to settler so I can actually enjoy the game without having to worry about a diplomacy system that's stacked against me and (often) follows no rational pattern whatsoever. And it's even worse when I have to sit there watching an AI calmly march its entire army up to my borders and encircle my frontier cities, while my only options are to DoW & suffer massive diplo hits with the entire world or sit around & wait for the inevitable attack.
This doesn't seem like such a difficult issue to fix--
just weight past positive modifiers more heavily than recent negative ones! Could an adept player use that system to their advantage by befriending everyone & then slowly picking off the AIs one by one? Absolutely, but the current system doesn't really prevent that from happening anyway (in fact, it probably makes it easier to accomplish by speeding things up). At least with that fix, you don't have to worry so much about the long-standing friendships which you've worked for hundreds of turns to cultivate being done in by one leader who all of a sudden goes all schizo on you.
Or heck, how about this for an addition: offer a game option which turns war off.
"Quick Reply" indeed.