Just to get my point of view.
Maps in games (be it quake, operation flashpoint, fh2, unreal or microsoft flight simulator) are not representations of the real world. They are blocks of collision that the player moves through. The player is not playing by looking at a wall, he is playing by colliding against it.
when creating maps, you look at static objects like grey meshes (in your head). A wall has three purposes:
-guide the player
-block the player
-provide cover for the player.
When you design a map, you decide what the player can, and can't do. It is the priviligue of mappers to force you in to a behaviour, a path of movement. The mapper creates the decisions for you: go left, or right. You are constantly making decisions as a player, some of them are important, some of them are less important. Still they are there because the mapper designed them for you. This is why we dont want to alter this by destroyable walls here and there. If the wall is high so you cant drive over with tank, it is not because the "wall" is low, it is because the mapper dont want tanks driving there. You must take another decision, for example drive on the road where the enemy has AT gun, you are forced to take this decision, because the gameplay happens when you engage the enemy, not when you drive around in the fields.
I agree to some point that a wall that is stuck down too low in to the ground should be drive overable (?) but in all honesty our maps are quite large and open, you often have more than one way of attacking the enemy and it is often hard to know where the enemy tanks are coming from. I honestly dont believe the gameplay would be better if we had this "unknown" factor of players roaming all free in tanks. Mappers want to control the movement of players, and our modellers have made so many variations of hedgerows and walls (broken, intact, low, high, small gap, big gap, etc) so if the mappers have created a very long route with no exit, it is bad map design, not bad model design..
Please post screenshots of areas where you feel it is annoying to not have any choices to drive, where you feel to funneled into a path and have not an option to play it differently.