I think that the menu and the upgrade selection screen should be controlled by wasd keys as well as the arrow keys. You have the ability to change controls for the game before it starts, but you don't have the ability to change the controls once the game has started. I got really far attempting the mouse for example, but I realized that the game has only up, down, left, right places to shoot and also that the mouse doesn't follow your character, so I wanted to change my decision in the later levels but couldn't.
Also there isn't much feedback on what each of the upgrades do. Yes they were obvious, but making a game means making a game for people who are somewhat slower than others as well. There should be just a brief description of what everything does. I mean, in some games weapon would increase the amount of bullets coming from your weapon into a particular direction while in others it increases the strength of the weapon. For all we know it could have increased the range.
Gameplay wise this game does very well with difficulty and the graphics are appealing. There's nothing brag worthy but I enjoyed the game.