The AI in general is far too aggressive in Civ5.
This makes them feel like generic warmongering idiots, and is especially a problem on the high difficulty levels in the very early game. I had one Emperor game recently where Napoleon met me on turn 7, and turned hostile immediately on the next turn because my army was too weak. Too weak! Well of course its weak relative to yours, you jerkwad, you got half a dozen free units!
Worse though was a recent Immortal game where Elizabeth declared war on me with no warning at turn 20, without even going hostile beforehand. Turn 20! Of course youre going to win by doing that, you dick, you started with a bloody swarm of free units while I only have a couple.
And its a problem across the board with the leaders. Its not just the folks like Monty, Odo and the ever-fickle Catherine I even had Gandhi turn hostile on me extremely early in a game even on one of the lower levels. Gandhi! What the heck, isnt he supposed to be the peaceful guy?
So seriously, whats up? Whatever happened to the grace period of several dozen turns in previous Civ games? Sure some of the AIs should be aggressive later on, but letting them all turn hostile or declare war in the extremely early game just makes them all out to be total dickheads.
Especially on the higher levels, you need at least a small grace period have a chance at catching up to the AIs insane bonuses. Its supposed to be a challenge, not half the time an automatic immediately send all our free unit hoards to crush the human. What kind of fun is that? You may as well just add in a piece of code on the high levels that randomly comes up with a You Lose, Try Again! screen half the time when you start a new game.
Please, bring back the grace period to give some semblance of a fair game on the high levels, and make some of the AIs less aggressive to stop them all seeming like generic warmongering morons.