ORIGINAL: Mynok
Computers cannot think. They don't have personalities. They aren't human.
This is a good point. I have a good friend (who happens to post on this board as "gwheelock") that takes this "human interaction" part and puts it to good use. If a person EVER backstabs him in a game, his focus changes to the destruction of that player's chances of winning. He pulls out all the stops.
Typically, computer games never do this (although, it wouldn't be that hard to code in). But, many humans do. And, if another human is playing with gwheelock, s/he needs to know that it is a singularly bad idea to backstab him early in a game.
Then, there's people like me: I like to try to play the game historically. As Russia, I hate the Turks, and I use the Prussians. And, I don't care much for the Austrians, either, but I'll play nice with them. At least early. GB had better think long and hard about making Russia an enemy (THIS Russia has a long memory!) France? I hate him, but if someone closer goes against my wishes, sure, I'll join him. Fully expecting him to backstab me at some point, and preparing for that day.
As GB, I want to control the world. "Make the world British. SIR!" That means controlling all of the other nations, at least partially. With money, I buy Turkish favors. With money I assist Prussia, Austria, and Russia against those disgusting Frenchies. All other nations exist to serve the empire upon which the sun never sets.
As Spain, I just want to be left alone. And, of course, in control of Portugal, Morocco, Naples, etc.

Eventually, France and GB will recognize the threat I present, but I just hope the rest of the world has grown up enough to take care of them for me.
As Prussia, I hate Napoleon. Not "I hate France". I hate NAPOLEON. With a bitter and undying passion. Nothing else matters. France is fine, as long as Napoleon isn't there. Oh, and she should get back to her 1796 borders, too.
There are other players that can be best described as "erratic". These are the ones who go to war at the drop of a hat. Computers can sometimes emulate this, but computers don't also PLAY erratically. It requires erratic play to be successful with erratic war declarations.
Some players are superstitious. To a computer, this would be "random chance" factoring in. But, there's this one guy: He tosses a handful of dice before a roll, and then picks out the ones that are sixes, figuring they are on a roll. The only problem is, it seems to work. His last name sounds similar to Heisenberg (of Physics' "Heisenberg Uncertainty Principle" fame). We call this fellow's dice rolling (replacing Heisenberg's name with his name) "The Heisenberg CERTAINTY Principle". This is because while his dice rolling is normal most of the time, when a critical event occurs, he can almost always roll a six at just the right time. We've nicknamed HIM "Certainty Principle" because of this.
Some players are brilliant planners. Some are brilliant strategic thinkers. Some are brilliant tacticians.
I would think that the perfect AI, would incorporate some of the above:
- Brilliant month-to-month planner
- Brilliant strategic thinker
- Brilliant tactician
- Lucky
- Unlucky
- Manipulative
- Historical
- etc.
Unfortunately, I fully realize just how hard such things would be to code. But, that's what humans bring to the table.