One effective way to reduce the ability of the tank is to make it suseptable to counter attack. Reduce its defense rating, increase its cost and place infantry on the map that have a fairly balanced attack and defense with both offense and defense AI flags checked. Also, to decrease the AI's reliance on tanks make infantry invisable and able to detect invisable units. This has effects on infantry able to detect subs near land masses, but subs are made for deep seas anyways arn't they? This way infantry units can substitute an effective surprise strike force on mechanized units, and can only be seen by other infantry units. The AI will build them because for some reason they love invisable units (don't ask me why). This is a good temporary fix until PTW comes out, but somewhat unrealistic as large infantry formations are pretty easy to detect. However if it was a scenario with a more complex combat system (i.e. a veriety of infantry units representing smaller formations) then it would be fairly realistic. Of course this makes special forces units obsolete since every infantry unit is invisable. Of course as a balancer infantry can only move one square at a time and if you want to get them quickly to the battlefield you'll have to load them into ground transports which arn't invisable and suseptable to attack.
Another counter to armor is artillery. Increasing the range of artillery and lethality can turn the course of war if they are strategically placed. A bombard rating as effective as the modern armor attack would, I beleive, influence the AI to build more artillery and further reduce the numbers of armor in game. The same could be said of rockets, missiles, and cruise missiles.
Its actually pretty simple controlling the AI. All you have to do is make it so that no one unit seems to tower over the rest, and providing unique abilities in a specific unit that can't be substituted in another. The hard part is making all the units unique or equal without compromising gameplay. However if a unit can't defend itself then the AI won't send it out alone, and stressing combined arms tactics is very realistic. The one major disappointment however is the lack of multi class AI strategies. If I want to make a land mobile anti aircraft vehicle, I can't check the air defense flag in the AI area because its a land vehicle. But thats a different chapter.
I want to add partisan/guerilla units, but I'm unsure if the AI will be able to use them.
) that I really can't say. I'll have to pay more attention to that. 
