One of the most common failing of AI is not bypassing strong points on the way to an objective. No civ can afford improving land with 2-3 cities built. It is insane. Idiotic Wonder policy. Well no one has possibly over-looked this one but settlers are still quite stupid.
AIs can build irrigations anywhere.
When AI units are within the vicinity of your city, you can't use the production from that square in the city screen, regardless of whether this is a friendly or hostile unit. You also rarely get the option to tell them to move it through diplomacy in a Democracy, therefore the only way to get them to move is to kill it - which again is usually impossible on Democracy and damages your reputation. It's really dumb when the computer opponents move their capital so often. I assume it's because they lack technology for other city improvements, but it's still dumb. In the game I'm playing right now each other civilization moves their capital once per turn at least, sometimes more.