A mixed bag
The graphics were simple yet very professionally executed.
The unit behaviour, animation and effects were also excellent.
But the gameplay definately gets a "nothing too new or interesting" from me. It's a shame because the combat is so cinematic and well executed, just a little more thought and effort on gameplay would have made it a _lot_ better.
Some suggestions:
Make it side scolling. Perhaps after each wave have the view window scoll to the right. Exploration is always good fun, and the variation in terrain might make it less monotonous.
Allow the player to select more than one unit at once. If you want to halt your tanks of helicopters in any location, you have to select them one by one, and they sometimes get in front of each other making it very hard to form a cohesive line.
Destructable scenery? I can't imagine that (relatively :P) it should be that hard (considering what you've already achieved) to add background objects like trees and simple buildings which would get blown to pieces!
If you went with the side scrolling idea, maybe have enemy defences? Attacking enemy bases could be fun and add variety and some more interesting tactics (maybe bombard from range with tanks).
Finally maybe make missions and a story line. For example you could have a starter mission with a limited number of infantry only, called scouting the area or something. The result might be you discover a base with a big missile launcher in it that you then have to assault in the next mission.
I don't mean to sound demanding, these are just thoughts for the potential of the game. The current gameplay really is very monotonous and simple, even within this genre (the artillery barrage just makes the game a case of building and launching one weapon). It lets down the excellent unit behavior and graphics, which you've obviously put a lot of work into!
I hope this is helpful / constructive.