I like the idea of tanks taking more damage from above (I suggested it elsewhere), but I'd say it shouldn't apply to mortars/arty, because the 'from above' bonus is already theoretically factored into the weapon's base damage (it's always supposed to hit from above... yes, well, people use mortars as rocket launchers, but that's another story...) Making them get a 'from above' bonus would just make them difficult to balance. Even better, make it just apply to infantry that are actually standing above you -- this would make positioning and the shape of the map more important, which I think are good things. Is it entirely realistic? No. But gameplay > realism, and it would behave, generally, in an intuitive fashion. Of course, another option would be to let arty get the 'from above' bonus, but then lower their base damage vs tanks -- so they only do good damage when you get a direct hit from above. This would limit the abuse where people drive arty directly under the enemy vehicle or whatever. But it might weaken them too much, because splash damage to tanks would be nerfed...
