Agriculture: I would seriously recommend everyone read Jared Diamond's Guns Germs and Steel, for reasons why nations developed the way they did, he posulates that it has to do with alot more than just 'Grasslands.' Which should also place a nation in the Sahel of Africa, India, Russia and China well within the food requirements. You need access to everything in civ, food just isn't going to cut it.
Military concepts in Civ have, always been lacking (but one shouldn't play Civ if they want such things).
Using your armies and navies you are not able to acquire land alone through said means. You must acquire (or destroy) an enemy's city in order to do so. And even then your military does not actually do that job, but culture does.
In a tactical sense Civ's AI has behaved similarly for quite some time. When you go to war with an enemy, at the beginning you will face a large portion of their army (holding only their defenders in reserve). You need to withstand this attack and then prepare for them to attack in a thin stream. On the attack armies are rather medieval, applying a spearheaded attack of brute force to a position (city), with little regard for manuever. There are no real fronts, only the aformentioned spearhead to take a city. Thusly the concept of a blitzkrieg attack is useless, as an enemy has no actual supply lines and control center from with they are being separated. A doctrine truer to Civ would be one of mass/concentrated assault, not one designed to pocket an army between an armoured spearhead and its infantry.
The one thing I always felt lacking in Civ was its (the world) dynamism. Short of war, the world is rather static (though the more vicious culture in Civ4 is nice). Very rarely did I ever see Civs fail on their own (AI egypt could in civ2 if she neglected to improve her tiles quickly, because of a food drain fear). I have often thought about how to represent real world happenings. For example some way to represent revolutions and civil wars would be great. The USA doesn't really have a way to exist within the 'rules' of Civ. In Civ2 if someone took the capital of a large civ it could split, but who took London?
I have always enjoyed Sid's games. I enjoy them for their vastness. There are other games which do well to show more narrow points in time or idea, but Civ does nicely showing a loose and open picture of the world.
Would everyone please keep it on topic as well.