Needs more visual effects.
I'm not a fan of the controls, but I can't think of any way to truly improve them, so I'll let that slide.
I think the major downside to this game is the mere repetition. You're stuck on a single screen, clicking random enemies over and over again. To my knowledge, there's no highscore system, either, so lasting a while doesn't wield any kind of reward, just more enemies to kill. Granted, I didn't play for an extremely long time. I just can't find the will to sit and click for more than five or ten minutes. I need some visual stimulation to keep me in my seat.