I know it's for 1-Game-a-month but should you feel like tweaking the game a bit, here are some comments that I hope you will find constructive!
- There is no X in the upper-right when you are in full-screen mode. No way to quit besides Alt-F4.
- When you kill several enemies in a short amount of time, you will see a succession of power-up screens. My idea would be to start a timer after a kill; if no other kills are made within a defined time period, show the pop-up screen. If more kills are made before the timer expires, then reset the timer and add one skill point. When the player ends his killing streak, show the pop-up screen with the amount of cumulated points to spend on the power-ups.
- It is very hard to judge the depth of the lasers, maybe a shadow or a lighting effect on the ground would help.
- Powering up the fire rate seems to increment linearly, which makes some upgrades feel useless. Maybe have the first upgrade shoot a bit faster, the next one 5 times faster, etc. Progression would be much more pleasant and have more of an impact that way. Also maybe a limit to how fast the fire rate can go.
- A powerup after each and every kill keeps the gameplay from flowing; it starts and stops very often. Maybe give a power up every 1, 3, 5, 8, 15 kills.
Keep developing!