I would love holding mouse button to keep shooting.
Also would like if the behaviour of the enemies was explained within the game the first time they are introduced. On level 12 and still not sure whats the blue things behaviour.
EDIT:
Nice surprise on lvl 3*, maybe there were more.
I would also love it there was a pause when you load into a level, waiting for your input to analyse the level.
I feel like i die at least once or twice every level because i expect it to start in paused state.
Cosy guitar song is nice.
Difficulty progression of the levels was not great, though i myself commit that error with puzzle games, im not much of a puzzler :D
Boss was pretty well done, just missing some epic organ music and some screaming when he dies.
now ill turn my sling into a guitar and make a duo...