Awesome arcade game!
Nice introduction, addictive gameplay, funky item descriptions in the shop. The difficulty curve felt a bit steep but that might be because I'm not used to hardcore games. Wasn't frustrating though (apart from the fact that you start in the beginning - I wish I had a checkpoint).
As for the pickups, you might add some indication when they are about to disappear (like flashing or something), that would prevent one from going for an item only for it to disappear.
Can't add much to that - the game feels polished and balanced, great job!