Great feedback. Really helpful stuff there.
I couldn't do a lot of that stuff because of time constraints. I was going to make the enemies increasingly harder and have the score roll over, but just didn't have the time. So, I opted for the quicker and easier way to add difficulty to combat. I left out music to make the download size as small as possible, so people wouldn't have to wait long to play.
I'll definitely try to implement these when I have some spare time.
I really like your idea of having different combos enabling specific attacks. I've added that to my todo list :)