Bit rough around the edges at the moment, but I suppose that's to be expected for a demo.
Certainly, you've got all the key pieces for building a good shooter: you've got the basic game logic of a wave structure with shop inbetween, you've got a basic variety of enemies, you've got basic collisions and destructable objects, basic amount of bullets/guns, and so on. It's all there, and that's certainly impressive in its own right, but it's just very basic and unexciting at the moment, as well as being a little clunky.
It's difficult for me to tell what you already know is lacking, but there was a general lack of feedback and sounds in the game, making it overly boring and muted when shooting enemies or exploding barrels and such. Hitboxes and collisions were occasionally very weird: a lot of the time my bullets would just collide against something in the environment that I felt they shouldn't or something I feel like they shouldn't collide against at all, like the fragments of debris left from a crate. Enemies bullets would appear awkwardly as well, farther from the enemies that I would expect them to. It also felt really unprofessional in how the game doesn't pause when a level is complete and you're in shop: every click has you shoot the characters gun. And so on.
Additionally, I could forgive the game all of its bugs and jank if it had a fun or unique concept to showcase, like some sort of cool movement, neat abilities, or all-new game mode or something, but there isn't anything like that from what I could tell.
At its current state, I'd say it's more like an alpha instead of a demo since it's pretty much just a proof-of-basic-game-engine instead of proof-of-fun-game-concept, but hey, you've got all the building blocks you need to potentially build a great game out of this!