I personally love this game. The simple style gives it a nostalgic view, while creating a strong atmosphere. The use of colour is well done, it gives a feeling of being nowhere, far from civilization. The platforming was pretty good; even though it was simple, it's clear that the game wasn't about agile dodging but exploring. The continuous use of infinite maps makes it feel well thought out. You used a lot (maybe too many?) save points, and the key collecting was really cool, although as someone earlier pointed out, showing on the map the keys you've seen might've been a nice addition. The hearts were cool too, but maybe giving them some kind of (secret) purpose would make it more rewarding for the player. (The different usage of blocks only for the hearts is great!) Like other people also pointed out, maybe making the character a bit faster might be a nice change, although I did not have a problem with his current speed. Not much to say about the map, other than well done. The music is really fitting, and the overall increasing glitchiness of both the music and the environment really builds towards the ending, which I personally love; it gives the feeling of a cursed place, and the pointless pursuit of the ultimate treasure.
One question (out of curiosity): why did you remove the achievements? I remember there being two or three.
Long story short: I really like your game, keep up the good work!