To start with good words first, yes I like the artstyle too, simple but sympathic! The gameplay is classic and nice and I think the upgrades were good as they are. I felt the differences and overall I think it is a nice job you did!
Can it be that you lose your upgrades if you die too often? At least I think I lost my fast shooting rate after two deaths. Apart of that I have to admit that I struggle to find the idea of death = progress here. Hit seems to be progress by recieving upgrades but death seems to downgrade (if it wasn't just a bug or temporary upgrade)?