Good points:
- There's a lot of content, especially with the upgrades
- There are loads of animations/scene transitions/special effects, which make the game exciting to play
A few suggestions:
- Audio felt very loud, had to turn down volume - add a settings menu so people can adjust volume to their liking
- Change the intro to something story-based instead of just promo
- Incongruent UI with some being smooth vector graphics and others pixellated. Nothing wrong with either, just choose one and don't mix the styles
- Put the player's health bar/status effects at a fixed position on the screen, as it's hard to see them when moving around if they follow the player
In general there's just a load of different styles being used in the game, making it feel unpolished despite the animations and juice.