This art game has the perfect mixture between storytelling, musical/graphical art and video game challenging play. I don't think there is anything negative that can be said about this game. I can't find anything to say other than it is close to perfection.
Maybe it can be unforgiving and/or frustrating to casual (or just plain bad) players trying to achieve the secret ending. People may think it should be easier. I prefer a thousand times the way it is, though.
An epilogue would be nice, too. Not entirely necessary, but I figure after so many retries for the clock challenges, people may forget some parts of the plot due to incredible concentration required during dark parts.
Speaking of: People downvoting the game saying something like "return trips are useless" simply can't comprehend what this is about - or are illiterate.
Anyway - very well done.