Just played through the game a couple of times, here's some thoughts:
First, the game is gorgeous in its presentation. It looks beautiful and sounds great. Never before has climbing a tree felt like such an epic journey. Movement also feels very good, and the multiple jumps make it easy to recover if you make a mistake.
I think the only area that this could be improved is the whole "puzzles and outsmarting enemies" thing mentioned in the description. While I enjoyed the different platforming challenges they really didn't require much in the way of thought. Run or blast enemies, jump to the next platform. Not much of a challenge or puzzle. I tried the game on hardcore mode, too, and didn't notice much difference.
A couple of other suggestions: First, the game felt even better after I mapped a controller to go with it, so if you can include controller support natively that would make it even better. Also, it would be nice to be able to skip the opening cutscene. It's lovely, but after seeing it a few times I just wanted to get into the game.
Last thought: The ending was cool, but after seeing it twice I still have no idea what it meant. That's not necessarily a bad thing, I just wanted to say it because I'm still scratching my head and thinking about it.