It's a definitely solid prototype, though as it is it deserves refinement.
Here are my thoughts and suggestions.
- A speed increase. While the controls are responsive, the movement feels a tiny bit slow. Frameskipping will help this, though it could also be adressed at the physics level. This is a delicate issue that I'm sure you've spent a lot of time tweaking.
- Make me want to use the secondary weapons in combat. This means making them slightly overpowered, as I need to make the choice to spend ammo on shooting when I could just use plasma. Personally, I would also have them fired on a second attack button, so I can easily switch between plasma and the secondary I have selected.
- The best part of the tutorial (IMO) was finding secrets and how it taught you to use fire on hay. The gameplay should more properly imply/teach the combat role of the secondary weapons (the dash's attack value too), essentially forcing you to use them against enemies. After ponderance, I think the missile would be effective against distant enemies and the fire against crowds, but I would have liked to learn that organically.
Probably remedied with more levels in general.
- Challenge in general. I am aware that this is like complaining an appetizer platter should have more protein in it, so instead I am going to wait patiently for a later release when the game actually has a difficulty ramp.
- Feedback for pressing up and down. Cave Story has good visual feedback for pressing those keys - of note is Down (used for interaction when on the ground) turning the character to look away from the camera and placing a ? over their head.
A little aiming would be nice, but it's also important in your design and I'd understand if you don't want to implement it.
Overall, a solid demo for what it's worth with a lot of potential for future challenges.