There are many things about Civ 5 that i really like and others that i don't like much at all, i can live with all of them, however there is one thing that pretty much ruins the game for me personally and that is the "ai plays to win" philosophy, and even then only because it does'nt work, if it worked well i probably would'nt have a problem with it.
I would'nt expect the AI to try and lose, but if it was really trying to win at any cost it would not declare war on it's liberator once it had been wiped out, it would be all smiles and bide it's time until it was strong enough for one of the victory conditions again, it may even join you on your wars and ask for cities which it could use to improve it's position, it should, for the most part, be friendly to all Civs in the hope of getting back to it's former glory.
I'd like to put forward an idea if i may, make the Ai feel that a high score at the end of the game is as important to it as the other victory conditions, that way it would be free to honour it's friends whilst still having a shot at at the title as it were, if it values high score it will be in a position at some point to go for any of the victory conditions.
I really miss the ability to be friends from beginning to end with a Civilization, the current model has removed so many random stories that used to pop up in my games, i currently know for the most part how every Civ will react, with violence and ingratitude, frankly it becomes predictably boring.