Not my favorite zed slayer.
The problem I had with the first ZitS games is that they were inanely hard after a few waves, and all the upgrading, repairing and such became unmanageable. This game seems too far a step in the other direction - it's zed slayer steamroll central. The game is never particularly a challenge and I didn't die once, get stuck once or even run out of ammunition, grenades, or first aid kits. However, keep in mind I play a lot of zombie games - I generally know how to draw out a horde, clear a room, and maximize my ammo expenditure. For the average player, it might be quite hard.
Enemy variety, enemy health, could've helped a lot - I personally found the super fast zombies highly annoying rather than frightening or hard to kill. Biggest issue you need to fix is a whole lot less powerups and ammunition. This sort of game is pretty cathartic for people who've had difficulty with the first games, but it shouldn't be such a pushover.
The grenades were virtually useless when the shotgun worked as well, the assault rifle had no real weaknesses and the pistol was basically useless with so many ammo drops. The upgrades made the zombies paltry resistance and I never used most of the items. The flare didn't seem to be very effective in this game, since it's primarily focused on advancing - perhaps making it slow down zombies would make it more useful?
Finally, the game is too damn short. You clear a couple blocks in a town, clear a building, repeat, then wipe zombies. That's literally it. With fantastic gameplay mechanics such as the helicopter passes, room clearing (freaking awesome) and exploring the city, you could have gotten so much more content out. I was f*cking pissed when the game didn't end with an epic vehicle scene trying to avoid the bombers. Why not add a few more buildings to explore? A barricade part, or something like that?
It's a highly enjoyable game with interesting mechanics but I can't help feeling there was a lot of wasted potential here.