In addition to all the suggested bugfixes, you should retain at least some money when you die. Between how expensive upgrades are, how little money you get from enemies, and how quickly you get killed, having to restart from scratch is highly annoying.
Okay, twice now I've had a zombie take a full magazine to the face from an upgraded fourth weapon and not die. I'm guessing there's a glitch in there somewhere.
Either friendly fire needs to be disabled or the flamethrower guy needs to have a much smaller area of effect (and/or better AI) because that one dude is responsible for like 99% of the damage I'm taking.
There's a split second pause between jumping and when you're allowed to throw your head (at least while using arrows + space) that makes some coins unnecessarily difficult to get.
The hit detection on those hot rods seems wonky. I get that you're not supposed to shoot the armored back, but they rarely seem to take damage unless hit directly from the front.
I think you should increase the no damage bonus as the game progresses. Not a lot, maybe +50 coins every four or six levels. Basically, something to make it less irrelevant by the midpoint.
It's an interesting idea, but the hit detection is too arbitrary. It's also very easy to get pinned to a wall by "corpses," especially when you have a large number of followers.
Okay, that worm that wanders around eating your units? Cheapest bullshit I've ever seen in a TD game. At least give me a chance to kill the stupid thing.
Do I correctly remember being able to manually reposition the ninjas in the last game? Either way, that would be very helpful here. I've frequently taken unnecessary hits because the ninja killed the last visible enemy on one side of the screen, only to be unable to reach an enemy spawning on the other side before it got several hits in.
Oops, I guess I should decrease the 'hold' delay on that. It didn't seem like a problem when I was testing. EDIT: I decreased the 'hold' and moved it up and made it fade faster... shouldn't be a problem now (v1.01k)
The endgame is pretty badly balanced. For one thing, the enemies seem to go first about 90% of the time, which isn't so bad early on, but when three two-shot-per-turn ships become standard encounters, it causes you to hemorrhage money without any real recourse, especially after buying the more expensive (and more expensive to repair) ships. The final boss is a joke. Even if you fight it normally, it's just one ship. The random encounters are more dangerous at that point, and had been for some time, in fact.
Having powerups drain over time is kind of a slap in the face considering the frequent (long) pauses between waves. If you really want that feature (to make the Indomitable feat more difficult?), you could at least slow it down. Like, by half. That would go a long way towards addressing the complaints about needing drops to survive.
If I am going to change the need for powerups I am leaning towards reducing the rate at which they drop and making the player stronger in general, like troglodytic suggested. But I have no definite plans right now.
The Ace achievement in Into Space was a pain in the ass. This one is basically impossible. Speaking of Into Space, this is pretty much the same game, just with cutsier graphics and less interesting upgrades.
You know, if the Enforcers would quit stopping me every other hour to search for contraband, they might be able to put a dent into the eight billion petty thieves per road.
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 "Road of the Dead 2"){kind=icon}
 "Space Cyborg"){kind=icon}
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 "Madness on Wheels"){kind=icon}
 "Learn to Fail"){kind=icon}
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 "The Bravest Hunter"){kind=icon}
 "Incursion 2: The Artifact"){kind=icon}
 "Endless War 7"){kind=icon}
 "Ninja and Blind Girl 2"){kind=icon}
 "Highway of the Dead"){kind=icon}
 "Dark Reality: Two Doors"){kind=icon}
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 "Frontier"){kind=icon}
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Thank you for your feedback. You can suggest ideas for the sequel.