I know that it isn't the biggest problem with the game, but the numbers on the guns seems to be too unclear. The difference between a 20 fire rate and a 30 fire rate seems like the difference between 4 bullets a sec and something like 30 bullets a sec. It makes the DPS numbers given for the guns pretty useless, as well.
Also noticed that the job title achievements don't update until you close and reopen the game. Would like to get a gallery for the dates as well. Maybe some different music? Agree with those who say that the numbers/progression is the most important thing to balance to make this a great game.
I want to reiterate that the comment about keeping the mouse button clicked down near the pipe after the mouse control level works. It only stops on the zoomed in level, but by that time, you should have no problem getting the hard badge.
I think there's too much grinding, but that's pretty much par for the course. Leave the game on overnight on a level that dribbles out enemies at a manageable rate, and you might get a decent amount of gold when you wake up.