One bit that sprang to mind:

During the whole sequence when Veers looks through his periscope, orders maximum fire power, shoots and destroys the shield generator, here are some ideas:

(1) try to include not just the dead AT-ATs in the background, but multiple burning holes where snow speeders have been shot down. Maybe extra rebel trooper bodies on the ground too if possible.
(2) do something with Veers periscope view, so it actually targets and shows four circles as to where the four AT-AT guns should be fired.
(3) when the guns fire, have them actually "build up" a bit like the game R-TYPE and make the beams more intense (less red, more white -maybe even bright if that's possible)

Here is an example (visually this may be too much, but I refer to something similar to the audio effect):
http://www.youtube.com/watch?v=Cn5NzvM2kMA&feature=related
from 2:40 to 2:45

(4) when the shield generator explodes, naturally replace the explosion, but add a shockwave that may even push clouds away and/or hint at the shield itself collapsing. You could nix the weird AT-AT shot at the trooper right before the shield shot, and just have the trooper fall down from the shockwave.