This is a fantastic demo, and a few tweaks away from forming a killer game. The controls are about perfect. The ability to strafe was such a pleasant surprise. Levels are enormous for this platform. The intro overview shows that off, and provides guidance.
The simplest change that would make the biggest impact is more color. There's colored elements in the background - you've obviously handled scrolling that in. Tinting whole rooms, or decorating them with similarly-colored objects, would make navigation more natural. It would improve the at-a-glance identifiability of the already-present variety.
Enemies, same story. They have a great range of behaviors. The cliche of palette-swapping to show which thing they do is cliche for a reason. Having to guess whether a turret is going to fire diagonally, or even come at you, is holding back the combat, when movement and shooting are this tight.