Hey! I don't know how much experience in games you have, but there are some things I can suggest to perhaps make your game keep players in a bit longer:

1) While interesting, having the WASD controls move you relative to the current mouse direction doesn't execute that well. Again, it's pretty unique, but it doesn't seem to add anything and in fact just takes more time to get used to when everyone is already used to the normal "absolute" movement of WASD (W always moves you up, for example). In general, if something doesn't add anything or change the way you interact in a significant enough way, perhaps just stick with the traditional way of doing it (this isn't always true, just consider it here).

2) Reloading was a bit long.

3) Your audio and visual feedback for hitting the zombies was pretty good, so I knew what was happening there, but there was no real preparation for a wave starting - even if the player can be smart and be sure to stay in the center when killing the final zombie of the current wave, it would help to flash some text or exclamation points to give players a bit more urgency and awareness to the new threats.

Continue creating! It's great that you participated, and you learned something just by making and finishing what you did. : )