What I love:
The ability to run.
The ability to purchase more powerful weapons.
The ability to unlock doors to new areas.
The changing from day to night.
What I think could be improved:
The zombies can spawn literally right behind me which can be irritating. I think they should spawn a certain proximity from the player. It would also be good if the player had the ability to purchase a radar to tell where all enemies are spawning.
The zombies move way too fast. I think for the early waves the zombies should move slowly (like in classic Romero movies) but get faster and more aggressive as the waves progress.
I think intermittently (say every 5 rounds) you should face a boss. Plant the giant zombie here, and give it special abilities like the ability to throw projectiles at the player. Have the music change during a boss wave to something darker.
I also think that the time of day should change only at the completion of each wave. So for instance on wave 1 start at dawn, then get darker every wave until night falls on the boss round, after which it goes back to dawn.
It would be nice to be able to shoot the zombies in the head!
This is something for the future, but it would be nice if the player had the option to enter a saloon and then barricade themselves against the amassing horde, allowing the player a brief pause from the battle. Perhaps have it so that here is where the player is able to purchase upgrades, health, etc.
Finally, to increase the feeling of dread and tension, between each wave have the player able to read scribbled notes or hear recordings of the (now dead) villagers talking about how frightened they are about these creatures overrunning them, perhaps warning the player of certain things (like 'beware the moon' - a warning about the boss)