Has the diplomatic AI become less gamey and more realistic? To me, this is the most important thing because it makes the game immersive. I can't play a game where every game becomes a human vs AI game as soon as you start winning. That's an exaggeration but I want to be able to make friends and game long allies, since just fighting by yourself is simply tedious and always one sided. I'm thinking about buying the Humble Indie Bundle so I can get the two DLCs for 15 bucks, but I'm still not sure.
Frankly I feel the diplo AI has got a lot worse with the post-BNW patch - partly it's the ideology system, which late game tends to rule AI behaviour and turn game-long allies with a different ideology against you: that sort of overarching diplo effect makes it all too clear the AI is behaving a certain way because the code tells it to. Basically it turns me off in much the same way the Civ IV passive open borders and trade bonuses do - that, together with the actual listing of modifier values in the tooltip, makes it very hard for me to engage with Civ IV AIs as anything other than AIs, and Civ V now feels much the same, while I found pre-BNW Civ V had much more personality.
Beyond that something in BNW seems to make AI personalities far less distinctive than I recall them being before - there were once entire threads on the vanilla and G&K civs and their personality differences which characterised each in entertaining detail.
There are still differences - some AIs are more expansionist, warmongers are coded to warmonger, and civs have preferred victory conditions, but these are traits that affect their overall playstyle rather than their interactions with the player. The AI is now almost pathologically scared of negative modifiers, and if no one's backstabbing you or spamming you with missionaries despite your protests, you don't get any sense that Harald is more loyal than Napoleon, or that Isabella is a religious nut, or whatever. You can make game-long allies, and sometimes even keep them in the face of opposing ideologies if you have shared rivals, but then I never had any problem doing that in vanilla or especially in G&K, and in BNW the same sorts of alliances and diplomatic behaviour somehow feel more mechanical.
In vanilla I actually cheered at one point when I brought Catherine into a war and she nuked Borsippa, before we went on a joint attack with combined forces wiping out the remaining Babylonian cities. in G&K I felt I developed a relationship over several games with Nebuchadnezzar, who was always dependable and seemed to end up as my game-long ally initially by default, and later because I actively kept Babylon onside in later games. I get very little of that feel in BNW; one identikit game-long ally is much like another. Sometimes it will be Nebby, sometimes Harald, sometimes Harun.