In ROTK the catupluts/trebuchets shouldn't work (as my brother insists on screaming every time he watches it
). The projectiles from the trebuchets in Gondor should either, when they land, make craters of some size in the ground (instead of kicking up a handful of dirt) or roll. Similarly the projectiles from the orc catupults shouldn't follow the trajectories that they do (my brother insists that they shouldn't even work, but since I'm not sure about that I'll just go with they shouldn't fire the way they do).
As a whole the orc catupults don't bother me, but the trebuchet projectiles do (Gothmog should not have been able to move three inches and survive!
)