Two critiques.
1. It's not obvious enough that the black stuff is dangerous and not just the darkest background color. It blends in too well with the backgrounds.
2. Not sure if I should criticize the number of stalactites in jump paths or the scarcity of checkpoints; I kept dying before I even saw the second checkpoint. It was awfully discouraging; if I can't even get through this first segment of the game, there's no way I'll reach the endgame.