Not very good, but good attempt.
Only the W key (Which goes with the direction of the reticle) and the LB works, which is very bad, add a A,S and D key also to the movement system.( 0/10)
Poor sound effects and boring music also drag the rating down, perhaps go on a free site and get some scary zombie effects. (0.5/10)
Weapons are EXTREMELY boring and un-satisfying, they should have a kick and perhaps send the player back if the weapon has enough recoil, also replace the dots with actual bullets, copy and paste it from a non-copyrighted website or image, just don't be lazy. (0.5/5)
I do live the large area, not many games of this flash genre have big areas that spread, they're usually locked to one screen, this is great and allows for more evasive choices. (+1)
Overall, this game is in bad shape, but has a lot of potential, the seeds are there, you have now got to let 'em grow
I have encountered two bugs, one is when you die, if you click play again and proceed to restart, when you click 'Begin' or whatever the option is, it comes up with the scoreboard for the second time and it gives you a score of 0, like you were Insta-killed. I also found that the 'Next Wave' button doesn't work. (Bugs drag the rating down, depending on the bug(s) severity)
Suggestions & Idea's:
Survivor Mode: In this mode, you are a survivor, you must survive against the zombies, you start at area, perhaps your house/safehouse, ETC, you must find food, water and ammunition, make the areas bigger with enterable houses and areas, make the areas go on for awhile, I suggest making it huge, with zombie spawns, perhaps also add survivors, that you can talk to and either raid or persuade them to join you or give you resources. Perhaps add vehicles too, like cars or motorcycles, that you need fuel and parts for *cough, cough* DayZ :P
This idea is the best one, since it is completly original and has never been done before on NG... I don't think :P
Enterable areas - Even though the areas are quite big, perhaps make that you can enter other neighbouring areas, maybe not in the levels, but perhaps in a story mode or the Survival mode suggestion I made before.
Better AI - Simple, make the AI better, improve the pathing, watch a few tuts and you should be good. Also, maybe make a Survivor AI too, where they also have weapons and you can talk to them.
Zombie Originality/Abilities - I suggest adding in Zombie abilities or moves, of course you'd need to code in that the zombies would use 'em. I suggest adding a knock back ability for larger zombies, so they can knock you back and perhaps onto the ground, (OPTIONAL) then you would have a chance to be stunned for a short duration.
I also suggest you add in a zombie pounces on you, I apologise if I used the term 'pounced' incorrectly, but I assume you understand, then the zombie would start eating you, reducing your health and you would repeatedly have to press a button, space I suggest, to get him/her off.
You probably won't add this in, since you are new, but if you're up to the challenge, then I dare you :P
That is all I got, I hope you do good with the game and I wish you good luck. PLEASE msg me if you do make any big updates to this game.