I like the retro art style. However, there are some graphical elements I'd adjust - the background image too bright and together with those trees, it camouflages some powerups (especially ammo) sometimes - the "pixels" sizes are not consistent. I don't mind having finer pixel art objects in front of a coarse background, but the background alone has different levels of detail - the health display should be more present
I know it's just a beta and you might not have concentrated on content much yet. But still: What is the goal? Is it just all about collecting coins? Is there any variety in difficulty (e.g. end bosses or huge waves of enemies)? I also think the "action" could need some tweaking. Try having more enemies or higher rate of fire, but have them dealing less damage, so difficulty overall stays the same.
I like it, it have a lot to potential - try add some inertia, movement will be more fun with it. (like in asteroids game) - i dont like automatic weapons change, i like to chose which i used. - i like camera shake - i like many different enemies - maybe it was nice with some boss :) - but game is fun.