Was not expecting a total gem within this game. Of all the jam games I've played so far, it has the most gameplay out of any, which explains why you had no time to do any graphics work. It's even got a boss and a lengthy and difficult escape sequence. I had 411 deaths for a total time of 68:13 (a lot of those deaths and that time was during the escape sequence, that double right angle turn toward the end is absolutely brutal).
However, I would not be surprised if most people gave up on the game before they even realized it was a Metroidvania. The game feels pretty impenetrable until you get the ability to shoot, and before that the game is very open, but it is very difficult to get around. I went up first and wound up at the blue switch palace (having tried for a while to get past a difficult corner turn), and realized I couldn't activate it, so I just refreshed the game because I didn't feel like getting out of there. I think maybe limiting the space before you get the first weapon might have helped -- openness is great, but not when you don't realize that there's an easier way through those red balls.
There were some smaller things that I appreciated -- one of the simplest is that the checkpoints have a memory for the direction that you entered them. It's a subtle design choice, but makes the game significantly less tedious. It was also great to have those shortcuts, reducing backtracking and making the game feel more holistic. Though one thing that might have helped were some landmarks. Obviously, you were probably very low on time and didn't have any time for graphics, but everything is just pink, and even just some colored dots here and there would help the player find their way. It wasn't that big of deal since the game wasn't that large though.
It was also neat that checkpoints were so frequent. The game is pretty difficult and hitting walls is very common, so it reduced frustration. Except for the escape sequence, which was brutal.
Anyway, great game. Just make the graphics look like an Atari game and it'll make the presentation go from eh to WOW pretty quickly.