A big detriment to the atmosphere of the game is the "play to win" attitude of the AI civs. I prefer to see AI civs as NPC characters that shape the storyline rather than as algorithms.
I'm glad I stumbled onto this thread. The forum, insofar as I saw, is mainly filled with the idea that an AI created solely to try 'win' was superior to the predictable and transparent AI of Civ4. I am not saying that it isn't, since Civ5's AI has its own merits, but the paradigm comes at the cost of 'immersive play' done in the manner Thimble suggests. Prior to this, actually, I thought I was alone in my sentiments.
I play Civ to enjoy the storylines of a nascent peoples. My neighbours and early encounters lay the brickwork for that story--if my neighbours are douches, I can hold a grudge and later conquer them; if they are cooperative, I can exist happily as one state amongst many, trading techs and so forth, relying on collective security rather than the singular might of my own army. This is where I had the most fun in Civ4 and its variety of leadership personalities.
Civ5, however? In Civ5, everyone thus far has just been a douche waiting to strike and conquer more land and take your capital, etc. It's like I'm playing against an unchanging world of Genghis Khans, all stuck with the idea that a bigger empire is a better empire. This means that the game never changes. It means that I'm always playing to 'win' and never playing to just play. For me, that gets boring. Very boring. However fun Civ5 is currently, if EVERY SINGLE AI is trying to win by any means possible (usually conquest), I will not stick to it for the years and years I've stuck by other games (Civ4 FfH2, Crusader Kings, EU3, etc). My ideal solution would be a variety of AI still trying to win the game, but by different and distinct means that don't all get resolved on a battlefield.
Can't wait for the source code to get released to modders, seriously.