Thanks for your feedback!
The problem with the interface and aiming in the game is known. I'll fix that.
I think it's not possible to make Zune multiplayer games with XNA, although I'm not sure. Anyway, it's very unprobable that such a function will be part of Tank Skirmish.
I am satisfied with the balance of weapons' damage and price. Yeah, there is only one weapon that is able to kill a tank with one shot and it's very expensive. But in my opinion this ensures fairness in the gameplay.
I also find a dirt weapon a good idea. It is already planned.