The difficulty levels w/checkpoints were very well done, I was able to play again and get all the way through the game this time (on casual, better step up my game now ;{P .
Before I'd only played up through the end of level 2, now after seeing the whole game I really liked the moving npc aspect and the secret area in level 3 that seemed to cut across and through multiple parts of the level. Really strong design.
I found an effect for the secret area where if my ghost goes back to the entrance it gets covered up again with me inside of it, but moving back and forth shows me again. I'm guessing maybe not intentional if I'm still within the boundaries of the secret, maybe. I actually found it an interesting kind of mysterious feeling, kind of a cool effect where it seemed the level was closing up on me. Could be an interesting fog of war thing to have it sometimes re-shroud traversed areas with you still inside them so you have to figure out how to get out, but if there are hazards nearby could result in a death that feels unfair.