one thing that is different is Armour Protection Levels.
With easy a tanks armour is the same all round. On Hard the Front of a Tank is much tougher than the back. So ideally u should plan your attack to hit those tanks at the back.

I think also REaction times from SAM etc are quicker on Harder settings.
Also the way they engage, i.e do they engage if attack or engage if they see u.