Definitely a strong showing
There were some things that can get better. Scrolling the map with the arrow keys is a little annoying, at least an option to use WASD would be nice.
Unit queuing would be a HUGE addition to this game. Consider it for any future versions.
While the gameplay was very smooth and fairly polished, sometimes everything felt a little slow. This might just be inherent to the flash medium, but if you could do anything to mitigate this it would be a good thing.
A greater unit variety, obviously, is always a good draw in the RTS.
The upgrades were nice.
Overall you've got a very solid offering that's definitely entertaining. I hope you make a second version, it can only get better from here. Good work.