Honestly, the first thing that comes to mind is that I'm sick of zombie games.
I agree with Orymus on the visual style.
However, I did give your game a try. Before I start, it might be worth mentioning that this isn't really one of my preferred genres, and thus my responses should perhaps be weighed with that in mind. That said, here are my thoughts on the gameplay:
The combat doesn't feel very interesting to me: I just move left or right and mash the "hit" button; zombies die in a single hit, and there's nothing else to do.
I did encounter one zombie that took more than one hit to kill, I believe--in which case I found that combat became a little awkward, as there's no apparent means of back-pedalling while attacking.
There's little feeling of impact to blows: when hit, zombies just fall over, and the player flashes slightly with an easily-missed animation. This might perhaps be addressed at least in part by changes to the animations, and by including impact-flashes or a bit of blood-spray for the player.
Level-completion isn't terribly well sign-posted, and ads sometimes appear at jarring moments: I had one instance in which it seemed that an advert had popped up in the middle of gameplay; it was only after dismissing the ad that I discovered that I had (presumably) completed the level, as I was presented with an end-of-level screen.
Regarding player-death, I think that I'd prefer to restart at will (or return to the menu, if desired) than to wait for a countdown to complete, as is currently the case: let the player control when and whether they return to play.