It had pretty much everything
However it lacked the gameplay. Don't get me wrong, the game was really impressive from a game dev's perspective. It had all the elements of a commercial game but it lacked that flare and feeling of passion and love. If this was a commercial project I'd say ok, good job. As this is a non-commercial project I believe you could add or change a few more things.
But to be precise
1. Map generation. Yeah I know it can be a pain to generate a random map. An even bigger pain is to generate a playable random generated map that's not too short and passable. Creating an algorithm that would make a unique, craftily designed, well-playable and enjoyable map is very, very hard. You failed at that point, and I don't really blame you. Try making a map editor next time. I'm sure it'd work better than RG. Simply put, the levels were sucky.
2. Storyline. Well, there was no. Storyline is often the element that keeps the player motivated. To know what happens next, to see if their expectations meet the story or not. It doesn't necessarily have to be an epic dramatic and deeply philosophical legend rivaling Homer's odyssey, it can be funny and amusing as well.
3. Gameplay. It was pretty minimalistic. I believe this was what you were in fact aiming for, but in the end the style got overwhelmed by the boredom. The guns were all identical, the enemies similar, the bosses easy, the levels ugly. No music, no feeling went into this game as far as I can see. Really, this is a game made by a coder, not by a gamer. It had nothing that would keep you glued to the monitor. Little to no spark and addictiveness.
Nontheless, treat this game as a beta and move on. You could easily reuse this engine with a few changes to create a much deeper and more interesting game.
Cheers