Things I liked:
- It's very appealing visually, I just like the graphics
- Absolute commitment to the retro style: nothing seems to be out of place
- Unique idea and gameplay mechanic
- Interesting levels and enemy variety that's actually being gradually introduced throughout the game instead of appearing all at once in first level
- Lots of small but satisfying details, like enemies bouncing into each other and score rewarding good play
Things that need improvement in my opinion: I really don't know, unfortunately. It's the most polished one I've seen yet, great job.