Just got done reading the article Greg suggested. With how much everyone fusses about diplomacy I'm suprised this isn't passed around more.
Diplomacy and so many other issues are talked about in this article, it could shed light onto a lot of 'why'd they change THIS?!'
So diplomacy is designed to work differently than Civ4. Now, while I agree there are some variables in the XML that could be tweaked, it won't matter if you're still treating it like a previous Civ.JS: The main reason is that we wanted to go a different direction with diplomacy. Instead of having specific modifiers and numbers on the screen that affected relations between different players, we wanted a little more mystery and even more rationality behind the AI players. Religion was something that we think didn't really fit with that, because you could just send your missionary somewhere, convert somebody, and then they would be your ally forever.
We wanted you to have tools to affect diplomacy, but not so directly. It was just a completely mechanical system. We wanted something that was a little bit more mysterious. That was the main reason.
We didn't just say, "Okay. We're going to remove religion now." We evaluated ways of keeping it around and seeing if we could make something of it. But it was so tied to the diplomatic model that having that separated just meant that it wasn't going to stand up on its own.
We decided that we can focus on other areas of the game and make it better. It didn't fit within what our goals were for diplomacy, which is a pretty big part of Civ V.
There are a few exceptions - as in when you liberate a capital and their pissed at you. This is fixed with just a few variables or a different text string.
But all in all the diplomatic confrontations are fairly congruent. If you make deals with them, they'll make deals with you. If you always ignore all their requests and then go make a 'simple reasonable exchange', they're not gonna forget you said 'no' to them so many times.
I settled next to Germany, bought some land. They opened diplomacy and said 'Hey! Quit buyin our land'. I had the option of saying 'Ooops, won't happen again'. A few turns later I buy some more land. They open up diplomacy and say 'Thats it! You said you weren't and you did'. The next turn they canceled our 'pact of cooperation'. Before all that we had a fair trade of luxury resources - after that they wanted 3x of mine.
Again in another game I'm playing Greece. I'm out doing missions for city states when Japan starts picking on one nearby. So I defeat some of Japans military units, finish the mission, and negotiate peace with Japan. Well the city state in question happened to be closer to Rome, so Rome pulls up diplomacy saying "These city states are under my influence blah blah blah". I had no quarrel with Rome at all, but in my war against Japan and making allies with the City-State, Rome became aggressive.
Diplomacy is no longer linear and can't be treated that way.