Like many of the other commenters are saying, I think you got something really good here. The mechanics, graphics, audio and content are all superb. Obviously the main menu is placeholder, so I don't think it is really worth talking about. I found myself in one situation where it was a bit frustrating and I quit the game and restarted from a previous checkpoint - this was in the underwater part with the downward water torrents, I got stuck at the bottom and couldn't get out even after using the reverse time feature to exhaustion a number of times. Not sure if this is intended to be a part of the level where you simply need to quite and reload at.
I think one of the main things that is currently missing now is the sense of story/plot/lore. I think a little bit of plot building as you make your way through the world would take this game to the next level. The sense of wonder and exploration I get while moving through the interesting landscape would be complimented by small snippets of story elements that you see along the way - why am I here, what is the purpose of this struggle?
All in all, I think you have a serious winner on your hands here. Definitely looking forward to seeing your continued progress.