It was rather frustrating...
Make an option to continue or not is the FIRST thing I would say needs improving.
The customization was ok but I think you should choose different aspects of each gun you purchase, that would make your game much more adventurous.
Maybe add new enemy every two or three rounds... It got far too difficult far too soon.
A much wider area would be nice,
air-strikes would be VERY good,
satellite co-ordinations would be brilliant (Used once every round)
Mini-bosses every ten rounds,
Customize your pilot (Choose different aspects for him if he has to eject from the mecha in a last second chance, but don't make him too powerful, about a quater that of the mecha.
Weather conditions - lighting improves power, rain decreases enemy sight, night decreases view, snow cuts engine, etc.
Custom vinyl - put it wherever you want on the mecha (and if you do, on the pilot)
lol a lot of stuff needs to be added to fully satisfy me. You probably won't take heed to this review anyways.